12 



EMBRYOLOGY. 



b.d 



- n.d 



- k.b 



Tig. 4.— Diagram of an egg with the nutri- 

 tive yolk in the centre, The germinative 

 Tesicie (k.b) occupies the middle of the 

 nutritive yolk (n.d), "which is enveloped 

 in a mantle of formative yolk (b.d). 



layer. Tlie egg exhibits central differentiation, and therefore does 

 not assume a constant position when at rest. As in the former case 

 the yolk was polar in position, so here it is central. Such a condition 

 is never encountered in Verte- 

 brates, but it is characteristic of 

 Arthropods. 



In order to distinguish the three 



modifications, Balfour has made 



use of the expressions alecithal, 



telolecithal, and centrolecithal. He 



calls those eggs alecithal in which 



the deutoplasm, in small amount, 



is uniformly distributed through 



the protoplasm ; telolecithal, those 



in which it is accumulated at the 



vegetative pole ; centrolecithal, 



those in which the accumulation of 



deutoplasm has taken place at the 



centre. In what follows, we shall speak of (1) eggs with uniformly 



distributed yolk, (2) eggs with polar deutoplasm, and (3) eggs with 



central deutoplasm. 



It is now expedient to illustrate what has just been said by typical 

 examples, and for this purpose the eggs of Mammals, Ainphibia, 

 Birds, and Arthropods have been selected. We shall also frequently 

 recur to these in the presentation of the subsequent phases of develop- 

 ment. 



The egg of Mammals and of Man is exceedingly small, since it mea- 

 sures on the average only 0"2 mm. in diameter. It is for this reason 

 that it was not discovered until the present century — in 1827, by Carl 

 Ernst von Baer. Previously the much larger Graafian follicle 

 of the ovary, in which the smaller true egg is enclosed, had been 

 erroneously taken for the latter. The Mammalian egg (fig. 5) con- 

 sists principally of a finely granular protoplasmic substance, which 

 contains dark, fat-hke spherules and granules (deutoplasm), and 

 which is turbid and opaque in proportion to the amount of these. 

 The germinative vesicle (k.b) contains a large germinative dot {k.f), 

 located, together with a few smaller accessory dots, in a nuclear 

 network (k.n). The egg-membrane is called zona pellucida {z.p), 

 because it surrounds the yolk as a relatively thick and clear layer. It 

 is a primary membrane, for it is formed within the Graafian follicle, 

 by the follicular cells. Under high magnification the zona pellucida 



