No. I.] THE EMBRYOLOGY OF CREPIDULA. 153 



believe, not to the mesoblast covering the intestine, but to the 

 distal portion of the intestine itself. 



Heymons did not observe a secondary origin of mesoblast in 

 Umbrella, though he suggests that possibly in stages later 

 than he has examined, ectoderm cells may migrate into the 

 interior to form mesoderm. 



VII. History of the Entomeres. 



At the time when the mesoblast is completely separated 

 from the entoblast the latter consists of the following cells : 



Macromeres (Basal Quartette) 4 



Smaller Entoblasts (Fourth Quartette) . . 3 

 Enteroblasts (Fourth Quartette) .... 4 



Total ... II 



We will now take up the further history of each of these 

 groups. 



I . The Four Macromeres. 



After the formation of the fourth quartette there is a long 

 interval before the macromeres again divide ; during this time 

 the nuclei of these cells become very large and vesicular, and 

 contain one or more large nucleoli. Figs. 52, Z6, Z"] . These 

 cells are composed almost entirely of yolk, and their nuclei and 

 protoplasmic portions lie near the surface just in advance of 

 the edge of ectoblast, and in this position they move around to 

 the ventral pole. 



What force is it which carries these nuclei around the ^g^ 

 just in advance of the ectoblast cells .-• If, as is sometimes 

 assumed, the initial polarity of the egg is due to the fact that 

 the yolk granules have a greater specific gravity than the pro- 

 toplasm, must it be supposed that the specific gravity of these 

 substances changes in the later stages of cleavage so that the 

 nuclei and protoplasm sink to the lower part of the cell while 

 the yolk rises to the upper part } The progressive movement 

 of the nuclei and surrounding protoplasm over the yolk coinci- 

 dent with the extension of the ectoblast would go against any 

 such conclusion, and would favor the view that this movement 



