DEVELOPMENT OF THE TWO PRIMARY GERM-LAYERS. 93 



numbers, especially around the whole periphery of the germ-disc. 

 Since they increase in number by nuclear division, and since 

 some of them, enveloped in protoplasm, become detached from the 

 yolk, they contribute to the continuous growth of the germ- disc, a 

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

 cleavage. The lower cell-layer, together with the whole yolk-mass 

 with its free nuclei, must be compared to the vegetative half of the 

 blastula of Triton (fig. 45 dz). 



The gastrulation proceeds from the posterior margin of the germ- 

 , and begins even some time before the egg is laid. The study 

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

 that, in the investigation 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 

 stated accurately before opening the shell. This results from the 

 following rule established by RUPFFER, ROLLER, GERLACH, and DUVAL. 



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 even during the process of cleavage. In the anterior region the 

 cleavage takes place more slowly than in the posterior half. Con- 

 sequently larger embryonic cells are found in front, smaller and 

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



The difference between anterior and posterior becomes more evident 

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

 thickened margin of the germ- disc (Eandwulst of German writers, 

 bourrelet blastodermique of DUVAL). 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 caused by the fact that the marginal ridge, in consequence of a 

 more vigorous growth of the cells, has become thickened and more 

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

 from its surroundings as a whitish crescentic figure (fig. 52 A s). 

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

 crescentic groove (Sichelrinne, ROLLER), by means of which the germ- 

 disc acquires a still sharper limitation behind. 



