34 EMBRYOLOGY. 



eggs are of considerable size and with few exceptions opaque, the 

 germinative vesicle, distinguished by its numerous nucleoli, undergoes 

 a regressive metamorphosis. As has been followed step by step in 

 Teleosts by OELLACHER, and in Amphibia by the author, it always 

 ascends from the middle of the yolk to its surface, and in 

 fact without exception to its animal pole : in the case of the 

 Frog (fig. 16 kb) this occurs many weeks before the beginning 

 of maturation. Here immediately under the vitelline membrane, 

 it becomes flattened to a disc-like body, being at the same time 

 somewhat shrunken. Further changes, which it is very difficult 

 to follow in detail, take place in a comparatively short time; 

 these occur in the case of the Amphibia at the time when the 



Fig. 16. Frog's egg in process of ripening. 



The germinative vesicle (kb), with numerous germinative dots (kf), lies quite at the surface of 

 the animal pole as a flattened lenticular body. 



eggs are detached from the ovary. For if one examines eggs which 

 have already escaped into the abdominal cavity, or have entered the 

 oviduct, it is uniformly found that the germinative vesicle with its 

 dots has disappeared. In this case, too, there are subsequently 

 formed from a part of the chromatic substance of the germinative 

 vesicle two polar cells and an egg-nucleus, as has been proved by the 

 fine investigations of HOFFMANN for some species of Teleosts, of 

 O, SCHULTZE for several Amphibia (Siredon, Triton), and of KAST- 

 SCHENKO for certain Selachians. 



WEISMANN and BLOCHMANN have discovered a very interesting fact 

 in the Arthropods. In eggs, namely, which develop parthenogenetic- 

 ally (in summer eggs of Polyphemus, Bythotrephes, Moina, Leptodora, 

 and Daphnia, as well as in Aphidae) only a single polar cell is elimin- 

 ated, whereas in eggs which require fertilisation for their further 

 development there are always two formed. At present, however, 

 this contrast cannot be established as a general law. For PLAINER 

 found that in the case of Liparis dispar there are formed in 

 parthenogenetic eggs, as well as in those which are fertilised, two 

 polar cells, the first of which again divides. BLOCHMANN arrived at 

 the same result from the investigation of unfertilised eggs of bees, 

 from which drones are developed. 



