282 



EMBRYOLOGY 



In case the egg is very rich in yolk, as in Rhynchelmis, there arise, 

 according to Vejdovsky, as the result of the first divisions, four blasto- 

 meres, from which four much smaller blastomeres are constricted off, so 

 that now four micromeres and four macromeres are present. While 

 the micromeres increase rapidly by division, the hindmost and largest of 

 the four large cleavage spheres buds off three cells of medium size : the 

 mesomeres. Now the macromeres also divide further ; the micromeres, 

 which, as well as the mesomeres, have meantime increased in number, 

 grow over the latter, which thereby come to lie inside. Between micro- 

 meres and macromeres a small cleavage cavity arises, which is soon 

 obliterated, when the small cells grow over the large ones further, in 

 this way forming an epibolic gastrula. 



Ordinarily several, nsuallj two, blastomeres are differen- 

 tiated before the formation of the two primary germ-layers, 

 but apparently exhibiting relations to both of them ; these 

 withdraw from connection with the other cells and enter the 

 cleavage cavity (Fig. 129 A). They constitute the fundament 



C 



Fig. 129.— J. to C, sections through embryos of Lumhricus trapezoides (after 

 KLBitrKWBRRG, from Balfour's Comparative Embryology). A, horizontal longi- 

 tudinal section of an embryo in the gastrula stage, which is about to divide into 

 two embryos : between ectoderm and entoderm the two large pole cells of the 

 mesoderm (m') can be recognized, with the mesodermal bands (ms) arising from 

 them on either side ; al, archenteron ; B and C, cross-sections of somewhat older 

 embryos, which show how the mesodermal bands (ms) move toward the ventral 

 side, and how the cavity (pp) makes its appearance in them. 



