the rudiment appearing after artificial insemination he 

 assumed to be a sign of death and the beginning of destruction. 



The rudiment gradually begins to be covered by the yolk; 

 in three to four hours after oviposition it occupies nearly 

 one third of the surface of the ovum; in seven hours, half the 

 surface (Figure 32, 3); and within nine hours, three quarters 

 (Figure 32, 4). After that, as in the rudiment, the division 

 takes place in the thin animal and vegetative layers, the 

 separation of the embryo begins, which at the beginning is not 

 distinguished on the surface of the ovum. Shortly after 

 formation of the embryo, the spinal fissure becomes noticeable 

 (Figure 32, 6) . The shafts ascend over the surface and move 

 nearer to each other, at the same time the very delicate 

 vertebral cord can be distinguished between them. At nineteen 

 hours after oviposition the spinal shafts are high and the 

 fissures between them become deeper (Figure 32, 7 and 8). 

 By the end of the first day the back is closed and separation 

 of the primary vertebrae begins (Figure 32, 9 and 10), and 

 the spinal cord becomes noticeably thicker. The head at this 

 time is equal in length to the trunk, rounded projections 

 appear, formed by the cerebral vesicles. An examination from 

 the side shows formation of the transparent projection of the 

 eye in the middle of the cerebral vesicle. Somewhat later the 

 ear rudiment is seen; the eyes at this time are beginning to 

 bulge and the first rudiments of abdominal layers are distin- 

 guished. At the end of the second day the embryo becomes 

 pear-shaped (Figure 32, 13); its ventral part consists of 

 two divisions: the anterior is rounded and the posterior is 

 in the form of a curved tube (Figure 32, 14 and 15) . Then 

 the embryo begins to straighten (Figure 32, 16) . The separa- 

 tion from the yolk in different fish takes place differently. 

 In those cases where the mass of yolk is not large, the abdominal 

 layers envelop it; in fish with voluminous yolk matter, the 

 embryo from which the yolk sac hangs disconnects (for example 

 in selachians and batrachidae) . In those cases, where the 

 yolk mass is not large, the abdominal layers envelop it; 

 while in fish with voluminous yolk mass, an unfastening of 

 the embryo takes place and the yolk sac is attached to it 

 (for example in selachians and batrachidae) . 



The branchial protuberances are expressed less clearly 

 than in amphibiae, but they are divided also by four fissures. 



432 



