VIII 



AKTHKOPODA 



227 



During the period of development which succeeds the one which 

 we have just described, a process termed "reversion" takes place. 

 This consists in the development of the dorsal surface, which now 

 begins to grow more quickly than the ventral one, forcing asunder 





FIG. 174. A portion of a sagittal section of .[f/r/i'/ni /<//</// -ht/li ii'n, greatly enlarged. 



(After Kautseh.) 



c.coe, coeloinic sac in cephalic lobe ; c.gr, cerebral groove ; <:l, eamlal lobe ; <l, dorsal siirl'.-iri. : 

 /.<_-, fat-cells ; 0-10, the coeloinic sacs in the abdominal segments 0-10. 



the head and tail lobes, and at the same time forcing the mass of 

 yolk which occupies the interior of the egg downwards between the 

 two mesodermic bands. As a consequence, these bands, and the 

 overlying ectodermic thickenings which constitute the rudiments of 

 the right and left portions of the nerve cord, become widely separated 



ck 



ccoe 



coe 



a 



FIG. 175. Two views of embryos of Agdcna undergoing reversion. (After Kishinouye.) 



A, an early stage of reversion. B, a later stage of reversion. nV-, "''"'. the second and tifUi 

 rudimentary abdominal appendages. Other letters as before. 



from one another, and the very narrow band of ectoderm which 

 occupies the mid-ventral line between corresponding thickenings of 

 the right and left sides becomes enormously stretched. 



This process is one for which it is very difficult to find a 

 mechanical explanation. Mere preponderant growth of the dorsal 

 surface will not account for it. This by itself would only lead to the 



