Studies on a Marine Viviparous Teleost, Ditrema temmincki BLEEKER―III About the Maturation and the Seasonal Cycle of the Ovary

In Sasebo bay the many ovaries of Ditrema temminck are collected monthly and observed their structure, seasonal histological change, development of their oocyte and growth of embryos in ovaries. 1) The ovary of Ditrema temmincki in one and their external form is different from that of the oviparous or ovoviviparous teleost, namely the oviduct part in the common ovary expands and forms their ovary, and there are two horny projection in its extreme point. 2) The weight of the ovary is minimum between July and November which the weight of the testis is maximum, the ovary begins to grow heavy in December and January, in March its weight increases suddenly, in April it shows a peak and in May it decreased suddenly and there is no embryo in ovary since the middle decade of this month. The spawning season of embryos of this fish in Sasebo bay is seemed from the last decade of April to the first decade of May. The highest period of the ovary weight is the lowest time of the testis weight. The highest and lowest time of the gonads of both sexes are difference for six monthes respectively. 3) The increase of the ovary weight in this fish is due to the growth of embryos in ovary, not by the maturation and the addition of their oocyte in ovary like in the oviparous or ovoviviparous fishes. 4) It is possible to distinguish their sex by the gonad in macro dissection immediately after the birth. 5) The number of oocyte is very few from the histological section of ovary in this fish. So the shape of their ovigerous fold is very different from that of the ovi-or ovoviviparous fishes and it is not branching form. And their ovigerous fold is not a greate quantity but is complicated winding form and the portion which the blood-vessel or oocyte has existe expands unusually, so their ovarian cavity is very rugged form intricately. It seems that the ovarian contents is a key to distinguish between ovoviviparous and viviparous. Untill the ovulation the ovary and ovigerous fold does not develop so much. The blood-vessel in ovigerous fold is a pair always by a little artery and a large vein which is full of blood-corpuscles. 6) The ovarian sheath which had discharged the embryos is very thick, and is able to distinguish from the ovary which has not experienced the parturition. 7) There is no nucleons stages on the progress of the maturation of oocyte in this fish. The first stage of oocyte is yolk vesicle or primary yolk stage. 8) Like in the ovoviviparous teleost there is no zona-radiata in the cell-membrane of oocyte in any stages. 9) The oocyte in the yolk stage has many oil globules and yolk globules, and the distinction between nucleus and cytoplasm become to be uncertain. When its maturation more advanced the columnar epithelium sorrounds in a layer on the outer side of the cell-membrane. And next the yolk globules dssolve and it becomes the full matured oocyte. 10) In the first progress of the ovulation the cell membrane of oocyte grows more thicker and separates from the columnar epithelium of its outer side, and the stroma cell layer which surrounds about oocyte and the epithelium of the ovigerous fold are torn and oocyte comes out to the ovarian cavity from the ovigerous fold. 11) There are many multitude of sperms in the histological section of ovary between the last decade of October and the first decade of November. this is the very same period to the time of copulation which had been presumed by the maturation of the testis in this fish in Sasebo bay. 12) The many multitude of sperms are porued in ovary in the shape of the sperm-balls, and after the sperm-ball creeps in to the slender projected part of the ovarian cavity which is formed by the matured oocyte in the ovigerous fold, the sperm-ball is untied. 13) The sperms are not maintained in ovary for the long time. After the copulation the firtilization is performed for a short time, and other sperms are consumed and disappear immediately after the firtilization. 14) The ovulated oocyte firtilizes at once. The contents of the firtilized egg contracts in morula stage, so the relaxation of the cell-membrane becomes a sort of the catch and the egg is not removed. It seems that the [ovulated oocyte is not implantated at all on the ovigerous fold. The embryos grow considerably in the ovulated place. 15) In the embryo which is larger than 5mm (the first decade of January) the yolk is not seen and subsequently it seems that the embryo absorbs the serum in ovary from the surface of the body directly. And then their rectum project to the outside in the stage of embryo like Neoditrema raneoneti, and there are many fold in its inside wall. It seems that the projection of rectum is one of the organ for the absoption of the nutrition in the stage of embryo. 16) There is no trophotaeniae in the embryo of this fish. 17) This fish does not superfetation, this is proved from the state of the ovigerous fold in the ovulation and the codition of the embryos in ovary. 18) The arimentary canal projects from the abdominal region in front of the anal fin untill about 40mm embryo in body length, and it makes a present of the condition of a sort of the physiological hernia. 19) The ovigerous fold consists of the two great lobules and is connected lengthwise slenderly with the epithelium of the ovarian sheath in back side of ovary and it grows with th embryos. The embryos are wrapped entirely by the ovigerous fold in several groups while the embryos are small, but when the embryos grew till about 40mm (the first decade of April) they begin to removed in ovary to easy place for the parturition. 20) When the embryo in ovary grows to 15mm (the last decade of February) in body length the dorsal, anal and caudal fin get bigger suddenly, and it is guessed that the absorption of the nutrition is performed by these fins, and all the margin of the menbrane between the soft-rays of the fin project especially in embryo from about 15mm to parturition. 21) The number of the embryos in a ovary is above thirty in two years fish and is about twenty in one year fish. The body length of the many embryos in a ovary is almost equal and the condition of the growth of the embryos in every ovary in the same time is almost constant, in a word the volume and weight of ovary in the same time is in direct proportion to the number of the embryo in ovary. 22) The body length of the embryo in the parturition is presumed in 55-60mm