170 



EMBRYOLOGY 



1). 



apart (Figs. 83 and 84). Thej lie quite isolated, as if they had 

 no sort of relation to one another, as is seen, for example, in the 

 thirteen-cell stage of Dendrocoelimi (Fig. 84). One would be 

 inclined to think of this as an abnormality if the observations 

 of Metschnikoff, Iijima, and Hallez did not entirely agree 

 on this point. The subsequent de- 

 velopment likewise proceeds in a 

 manner altogether original, its pecu- 

 liarities being evidently a result of 

 the large amount of yolk-substance 

 which must be taken up by the 

 embryo. 



In the further course of develop- 

 ment some of the yolk-cells are dis- 

 solved, so that the embryo now lies 

 in a finely granular protoplasmic 

 mass, in which some of the nuclei of 

 the yolk-cells can still be recognized (Fig. 84). The division 

 of the blastomeres continues, and as a result of it there is 

 produced a spherical heap of from seventy to eighty irregu- 



FiG. 82. — Cleavage stage 

 of two blastomeres (Be), 

 with, the surrounding 

 yolk-cells (Ds), of Deiidro- 

 C(£\\iM\ lacteiuii (after 

 Iijima). 



Figs. 83 and 8i. — Cleavage stages of DenürocceXurix lacteioii (after Hallez). In 

 one stage four blastomeres (Be), in the other thirteen, with surrounding yolk-cells 

 (Ds), which in the later stage are partly fused into a common mass. Their nuclei 

 (shaded dark) are still visible in this mass. 



larly arranged cells. Changes are soon manifested in this, 

 which result in the establishment of the germ-layers. Some 

 of the peripheral embryonic cells move to the edge of the 



