VIII 



AETHEOPODA 



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 Agdena labyriTUhica^ greatly enlarged. 



(After Kautsch.) 



ccoe, coelomic sac in cephalic lobe ; c.gr, cerebral groove ; c.l, caudal lobe ; d, dorsal surface ; 

 /.c, fat-cells ; 6-10, the coelomic sacs in the abdominal segments 6-10. 



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 



Fig. 175. — Two views of embryos of Agelena undergoing reversion. (After Kishinouye.) 



A, an early stage of reversion. B, a later stage of reversion. a62, abs, the second and fifth 

 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 



