Observations and Experiments on the Ctenophore Egg : 1. The Structure of the Egg and Experiments on Cell-division

Observational part: 1. The ctenophore egg is composed of (a) the outer homogeneous layer, (b) ectoplasm and (c) entoplasm. 2. The outer homogeneous layer is homologous with ZIEGLER\u27S ""hyaline Aussenschicht"" of the echinoderm egg. 3. The ectoplasm is an alveolar plasm and rays may be formed in it. 4. The ectoplasm is phosphorescent. 5. The sperm-rays and pole-rays of the first division enter the entoplasm, the alveolar walls of the latter taking a radial arrangement. 6. Polyspermic eggs may cleave normally. 7. Cleavage is not strictly unilateral, the furrow being formed, in the micromere region. 8. Fine spinning of the homogeneous layer can be seen at entrance and at the bottom of the cleavage furrow. 9. In the ""cleavage head ""radiations are seen in the homogeneous outer layer and ectoplasm, but they do not extend into the entoplasm. 10. Beneath the ""cleavage head"" the entoplasmic alveoles are considerably compressed. 11. The micromeres consisting almost entirely of the ectoplasm cleave very similarly to the whole egg. 12. In the beginning of each cleavage, ectoplasmic accumulation is seen at the macromere pole. Towards the end of cleavage, an accumulation appears in the micromere region. Experimental Part: 13. Enucleated fragments destitute of the ""cleavage head"" not manifest any division activity. 14. The cleavage plane is not predetermined in the egg. 15. An enucleated piece provided with the ""cleavage head"" divides by itself without the aid of either nucleus or centrosome. Nor is the ray system necessary for the cleavage of enucleated pieces. 16. The cytoplasm above the level of the cleavage head has little influence upon the accomplishment of the division. 17. The accumulation of ectoplasm over the micro- and macromere poles is formed in enucleated fragments in the same way as in the whole egg. 18. If the removal of the nucleated portions is done at the beginning of the division, the cleavage furrow goes on normally irrespective of the angle of the section. If, however, the same operation is performed upon an egg in which the cleavage has further advanced, the division plane is in most cases turned towards the side of larger amount of cytoplasm, the enucleated fragment being divided into nearly equal parts. 19. Sometimes in the egg operated on the cleavage furrow curls up towards the macromere pole. 20. If the ""cleavage head"" be split lengthwise, a new head"" forms nearly at right angles and towards the portion with larger amount of cytoplasm. 21. If an incision is made in the egg in the micromere region the cleavage is not affected