V 



The Germ Layers 

 i. Formation of the mesoderm 



At the time when the germ layers begin to be formed, (32-34 

 hours) the blastoderm presents three more or less distinct di- 

 visions: (1) a median dorsal strip, (2) the blastoderm of the 

 \entral and lateral surfaces, (3) two bands, one on each side of 

 the dorsal strip (1), between the latter and (2). 



The median dorsal strip (Fig. 18A, DS) extends to the caudal 

 pole, from a point slightly ventral of the cephalic pole. It is 

 widest near the cephalic pole, attaining here a width of about 

 one-third of the circumference of the egg in this region, caudad 

 of this point it narrows gradually and over the remaining distance 

 continues quite narrow (Fig. 18B). It is composed of a relatively 

 small number of extremely thin and flat cells, and also differs 

 from the remainder of the blastoderm in its close relation to the 

 protoplasm surrounding and permeating the yolk. As may be 

 readily determined in most preparations the yolk is enclosed in 

 a pellicle of protoplasm, derived from the inner cortical layer, 

 forming a part of the interstitial protoplasmic meshwork and 

 continuous with it. This pellicle is commonly separated from the 

 basement membrane of the blastoderm by a narrow space. This 

 relation, however, ceases at the lateral edges of the dorsal strip, 

 since here the protoplasmic pellicle unites and becomes continu- 

 ous with the lateral edges of the cells composing the strip. This 

 connection becomes especially evident during the formation of 

 the germ layers and the amnion. As a result of the disappear- 

 ance of the pellicle under the dorsal strip the latter supplies the 

 place of the pellicle and thus comes into close relation with the 

 yolk and the meshwork of protoplasm within it. The fate of the 

 dorsal strip will be dealt with later, but it may be said in advance 

 that it takes part neither in the formation of the embryo nor its 

 covering (amnion), and is therefore strictly non-embryonic. 



43 



