14 HISTORY OF THE EGG FROM FERTILIZATION TO CLEAVAGE. 



ceptibly exceed that of the egg sphere, while in Fig. 53 it rises somewhat 

 above the general surface. The second polar globnle is fully formed 

 about this time, but it never comes so plainly into view as the first. 



In the next, stage (Fig. 46 = 30 min.), which corresponds nearly with 

 Figs. 54, 55, and in which the pronuclei seem to have blended into one 

 spherical body, the first cleavage nucleus, new phenomena are introduced, 

 which announce the approach of cleavage. While the volume of the 

 disc continues to increase throughout these and following stages, it now 

 undergoes a marked change in form. The inner face begins to flatten, 

 while the outer becomes more and more convex, soon coming again into 

 contact with the membrane (Fig. 47). The polar globules are thus pushed 

 beneath the surface and are seldom seen again. 



In this egg the inner face of the disc not only lost its convexity, but 

 actually became concave (Fig. 48), the whole disc assuming the form of a 

 meniscus. In the normally fertilized egg, the flattening is not usually car- 

 ried quite so far, and is followed by a central indentation of the inner face 

 (Fig. 58). This indentation may be called the inferior cleavage groove, to dis- 

 tinguish it from the surface (superior) groove which follows in a few minutes. 

 Tbese inferior grooves figure quite conspicuously in the early cleavage 

 stages, and we shall see that the cleavage cavity has its origin in them. 



The formation of the blastodisc has often been described, and our 

 observations agree in the main with previous accounts. Very interesting 

 " amoeboid " phenomena are said to attend this process in certain eggs, 

 but nothing of this sort has been witnessed in any of the eggs we have 

 studied. Both the outer and the inner surface of the disc are perfectly 

 smooth and regular throughout. The formative protoplasm is clearly 

 separated from the yolk from the beginning, its inner boundary being 

 scarcely less sharp than its outer. Not the least trace of intra-vitelline 

 strands of protoplasm beneath the disc has been detected. That such 

 strands exist in some Teleost eggs, cannot of course be doubted in the 

 face of an overwhelming amount of affirmative testimony. 



4. Maeginal and Subgerminal Derivation of the Blastodisc. — 

 Ryder (No. 4, p. IT) has already called attention to the fact, that the pe- 

 ripheral segregation of formative material is sometimes complete, as in the 

 egg of the cod, and at other times incomplete, as in the eggs of the clupe- 

 oids; We have, accordingly, to recognize two modes of growth in the ger- 

 minal disc. In the one case, the disc enlarges by additions at its margin; in 



