Observations and Experiments on the Ctenophore- Egg. T 



As a digession, the results from the study of sections of this- 

 stage may here be given. Cleavage begins at the telophase of the 

 first mitosis. Rays (pole-rays) of a considerable length extend 

 into the entoplasm of both blastomeres, centering about the newly 

 formed nuclei (the centres are in all probability situated very close 

 to the nuclei). Besides, something like sheath-rays are found 

 between two asters. They are evidentl}^ cut apart by the growing, 

 " cleavage head." A similar condition has been observed in the 

 Hydra egg by Brauer ('91 Taf. IX, Figs. 16 and 17). 



Now coming to the next stage (PL II, Fig. 13). The bottom 

 of the depression has been carried farther down. Usually the clear 

 protuberances at the entrance of the furrow are drawn in. Fine 

 spinning is seen. The protoplasmic threads are not parallel to one 

 another. Often they decussate. At the "head" are radiations- 

 as in the foregoing stage. 



A fully formed " cleavage head " is shown in Fig. 14 (PI. II). 

 By this time protoplasmic spinning is restricted to the entrance 

 and bottom of the furrow. The " head '' is a thickened ectoplasm 

 Here one notices that the outer homogeneous layer also has in- 

 creased in thickness. Réfringent alveoles in the ectoplasm are 

 arranged radially as extensions of fine radiations of the homoge- 

 neous layer. Towards the entoplasm are processes (Zacken) as- 

 has been rightly observed by Ziegler ('98, '03). I tried hard to 

 detect rays extending from the tips of the processes into the 

 entoplasm, but contrary to Rhumbler's assumption ('99 p. 203 

 Fig. 12 and p. 205 Fig. 13), there were no such things; here the 

 ectoplasm simply comes in contact with the entoplasm. The 

 alveoles of entoplasm liere show a peculiar arrangement worth 

 noting. Those along tlie walls of the cleavage furrow seem 

 to have been carried down with it and those found at the tip 

 of the " head " are somewhat flattened (cf. PI. II, Fig. 28). This 

 undoubtedly shows that the "cleavage head" pushes downward 

 instead of being pulled l)}^ the contraction of rays, stretching 

 between the cleavage head and the micromere pole. At the 

 next stage the alveoles recede from the median plane as i& 

 shown in Fig. 15 (PI. II). It will be of some interest here to 



