CH. XIII] 



ORGAXS FROM THE EXDODERM 



143 



that the whole embryo assumes. The early enlargement of 

 the anterior archenteric cavity and the formation of a single- 

 layered wall at the anterior end, with the subsequent formation 

 of the gill-slits, would seem to be the result of the activity of 

 the endodermal cells of those regions. On the other hand, 

 some of the changes in shape that the lumen undergoes would 

 seem to be due to the change in shape of the whole embryo as 

 it elongates antero-posterioiiy, and narrows from side to side. 

 Nevertheless, even in this case the cells do not seem to be 



FIG. 40. Cross-section through the middle of an embryo (83 mm.). AR. Archen- 

 teron. Ms. Mesoblastic somites. X. Xotochord. Xs. Xeural crest. M. Medul- 

 lary tube. PR. Pronephros. Sx. Subnotochordal rod. So, SP. Somatic and 

 splanchnic mesoderni. (After Marshall.) 



entirely passive, for the number of cells lining certain parts of 

 the early archenteron is, in cross-section, considerably larger 

 than the number lining the same region at a later stage. 

 Either certain of the cells have pulled away from the surface 

 and have passed into the yolk, or else they have changed their 

 position relative to one another on account of the lengthening 

 of the archenteron. In the latter case the total number of 

 endoderm cells lining the archenteron would still be the 



