14 



EMBRYOLOGY. 



the individual components, as is the case in other Mammals and 

 lower animals, where one distinguishes with great ease granules 

 and distinct drops. The outer layer or peripheral zone of the yolk is 

 more finely granular and still more transparent than the central 

 part, and contains the gerrninative vesicle with a large germinative 

 dot, in which NAGEL was able to observe amoeboid motions. The 

 zona pellucida is remarkably broad ; it is striate, and is separated 

 from the yolk by a narrow (perivitelline) space. There are two or 

 three layers of follicular cells attached to the periphery of the egg 

 when it is set free from the GRAAFIAN follicle. The long diameters 

 of these cells are arranged in a radial direction around the egg, as 

 is general in Mammals, and it is due to this circumstance that they 

 have received the name corona radiata, introduced by BISCHOFF. 

 'The human egg without the follicular epithelium measures, on the 

 average, 0*17 mm. in diameter. 



The eggs of many Worms, Molluscs, Echinoderms, and Ccelenterates 

 agree with the Mammalian egg in their size, and in the method in 

 which protoplasm and deutoplasm are uniformly distributed through 

 the egg. 



The eggs oj Amphibia, which were cited as the second example, 

 form a transition from simple eggs, with uniform distribution of 

 yolk-material, to eggs with distinctly expressed and externally 

 recognisable polar differentiation. Already these have deposited in 

 themselves a large amount of deutoplasm, and have thereby acquired 

 a very considerable size. The Frog's egg, for example, is stuffed 

 full of closely compacted, fatty-looking yolk-lumps (Dotterschollen) 

 and yolk-plates. The egg protoplasm is in part distributed as a 

 network between the little yolk-plates ; in part it forms a thin 

 cortical layer at the surface of the egg. Upon closer examination 

 however, the beginning of a polar differentiation is most distinctly 

 recognisable even here. It manifests itself in this way : at one 

 pole, which at the same time appears black on account of a deposit 

 of superficial pigment, the yolk-plates are smaller and enveloped in 

 more abundant egg-plasm ; and also, nrobably as a consequence of 

 this, slight differences in specific gravity are distinguishable between 

 the pigmented and the unpigmented, or the animal and the vegetative, 

 halves of the egg. 



The germinative vesicle (fig. 2) lies in the middle of the immature 

 <>gg, is exceedingly large, even visible to the naked eye^ and multi- 

 nucleolar, inasmuch as there are a hundred or more large germinative 

 <lots (kf) distributed immediatrf? under the nuclear membrane. 



