DKVKLOPMBNT OF TIIK T\M i'i:i.M\i;v C.I:I:M i.v, 



numbers, especially an-uid (lie whole periphery of tin- g< rin di>r. 

 thev increa.-e in nimilu r ly nuclear division, and 



-'uit df them, n\viop <i in protoplasm, become ! t.-u-lml from ihe 



yolk, they contribute to tin- continuous growth of the germ-l 

 process which lias already (p. 65) been described as supplementary 

 ch avage. The lower cell-layer, together wit h the whole yolk 

 with its five nuel i. must be compared to the veg tative halt of the 

 blast ula of Triton (tig. -1.0 </:). 



The gastrulation proceeds from the posterior margin of the germ- 

 di>r, ami brains -\vn some time In -fore the eri; is laid. Tln->tmly 

 of it is coupled with great dillicultis, and demands, most of all, 

 that, in the invent iirat ion of the disc by means of sections, one should 

 be accurately informed concerning the position of its anterior and 

 posterior margins. The orientation is essentially facilitated by the 

 fact that, in the case of every Hen's egg, with rare exceptions, the 

 side toward which the front end of the embryo is directed can be 

 Mated accurately before opening the shell. This results from the 

 following rule established by KUPFFER, ROLLER, GERLACH, and Dr\ A i.. 



When one so places an egg in front of him that the blunt pole is 

 turned to the left, the more pointed one to the right, then a line 

 uniting the two poles divides the germ-disc into a half on the side 

 toward the observer, which becomes the hind end of the embryo, and 

 a forward half, which is developed into the head-end. By taking 

 into account this rule, one can establish a difference on the germ- 

 disc (ven during the process of cleavage. In the anterior region the 

 cl.avaire tak.> place more slowly than in the posterior half. Con- 

 sequently larger embryonic cells are found in front, smaller and 

 more numerous ones behind (OELLACHER, KOLLIKER, DUVAL). 



The difference between anterior and posterior becomes more evident 

 at the beginning of gastrulation. If one now examines carefully the 

 thick* md margin of the germ-disc (Eandwulst of German writers, 

 I'ourrelrt blastodermique of IMVAL), it is seen that the disc is limited 

 in front and on the sides by a notched and indistinct boundary, 

 but behind, on the contrary, by a sharper contour. The latter 

 is cans* d by the fact that the marginal ridge, in consequence of a 

 more vigorous growth of the cells, has become thicken ed and more 

 opaque, and has asMinied a whiter colour. It is distinctly recognisable 

 from its surroundings as a whitish crescent ic tigure (fig. 52 A s). 

 Often there is also observable in the creacenl a narrow furrow, the 

 > vntic groove (Siehelrinne. KOLLER), by means of which the germ- 

 disc acquires a still sharp, r limitation l.ehind. 



