NOTES ON THE DEVELOPMENT OF THE ARANEINA. 68 1 



ual thickenings themselves, though much more conspicuous than 

 in the previous stage' (PL 31, fig. 16, v. c], are still integral parts 

 of the epiblast. They are more widely separated than before in 

 the middle line. The mesoblastic somites retain their earlier 

 constitution (PI. 31, fig. 16). Beneath the procephalic lobes the 

 mesoblast has, in most respects, a constitution similar to that of 

 a mesoblastic somite in the trunk. It is formed of two bodies, 

 one on each side, each composed of a splanchnic and somatic 

 layer (PI. 31, fig. 17, sp. and so), enclosing between them a 

 section of the body-cavity. But the cephalic somites, unlike 

 those of the trunk, are united by a median bridge of mesoblast, 

 in which no division into two layers can be detected. This 

 bridge assists in forming a thick investment of mesoblast round 

 the stomodaeum (sf). 



The existence of a section of the body-cavity in the praeoral 

 region is a fact of some interest, especially when taken in con- 

 nection with the discovery, by Kleinenberg, of a similar structure 

 in the head of Lumbricus. The procephalic lobe represents the 

 praeoral lobe of Chaetopod larvae, but the prolongation of the 

 body- cavity into it does not, in my opinion, necessarily imply 

 that it is equivalent to a post-oral segment. 



The epiblast of the procephalic lobes is a thick layer several 

 cells deep, but without any trace of a separation of the ganglio- 

 nic portion from the epidermis. 



The nuclei of the yolk have increased in number, but the 

 yolk, in other respects, retains its earlier characters. 



The next period in the development is that in which the 

 body of the embryo gradually acquires the adult form. The 

 most important event which takes place during this period is 

 the development of the dorsal region of the embryo, which, up 

 to its commencement, is practically non-existent. As a con- 

 sequence of the development of the dorsal region, the embryo, 

 which has hitherto had what may be called a dorsal flexure, 

 gradually unrolls itself, and acquires a ventral flexure. This 

 change in the flexure of the embryo is in appearance a rather 

 complicated phenomenon, and has been somewhat differently 

 described by the two naturalists who have studied it in recent 

 times. 



For Claparede the prime cause of the change of flexure is 



B. 44 



