40 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



embryos resulting from meroblastic eggs, however, the archenteron is open below 

 and spread out on the yolk, and the hypomeres extend out over the yolk. The 

 differences between the two types of embryos are illustrated in Figures 8^4 and C. 

 The embryo in the case of meroblastic development is later constricted from the 



notochord 



notochord 



neural tube 



coelo 



entoderm 



intestine 



entoderm 



epimere 



mesomere 

 dorsal mesentery 



ectoderm 



entoderm 



FIG. 8. Diagrams to show the differentiation of the mesoderm in holoblastic and meroblastic 

 types of development. A and B, holoblastic type; C and D, meroblastic type. A , differentiation of the 

 mesoderm into epimere, mesomere, and hypomere. B, separation of the epimere from the mesomere, 

 appearance of kidney tubules in the mesomere, and closure of hypomere around the intestine to form 

 dorsal and ventral mesenteries. C, similar to A but in the meroblastic type, showing entoderm and 

 mesoderm growing around the yolk; note how embryo is spread out on the surface of the yolk; x, indi- 

 cates lines where embryo is cut off from yolk in making sections for microscopic study. D, similar to B 

 but in the meroblastic type; the entoderm has completely surrounded the yolk; the mesoderm has 

 nearly done so; the yolk sac is seen to be a part of the intestine; the embryo is partly constricted from 

 the yolk sac, the constriction being the yolk stalk. (C from Wilder's History of the Human Body, 

 courtesy of Henry Holt and Company.) 



yolk by the formation of deep grooves on all sides. The yolk then hangs from 

 the ventral surface of the embryo inclosed in a sac of blastoderm, the yolk sac, 

 which is connected with the embryo by a stalk, the yolk stalk, as shown in 



