METAMORPHOSIS. 



29 



here has changed into a plate formed of deep cells which becomes 

 invaginated (Figs. 11, r and 12 A, a). The sac which thus arises 

 soon becomes completely cut oft' from the body-wall (Fig. 12 B\ 

 and represents the rudiment of the ectodermal and also of the 

 entodermal parts of the polypide. A second cell-layer (!>} appears 

 on the outer side of this sac, this being the mesodermal layer 

 of the polypide. Yarious conjectures have been made as to the 

 origin of this layer. According to BARROIS (Xos. 7 and 9), there 

 are, in Lepralia, two ectodermal thickenings at the sides of the 

 anterior ectodermal furrow (Fig. 10, x) ; when the corona bends 

 over, these reach the interior of the primary zooecium, and they 

 alone are unaffected by the subsequent degeneration of the larval 



FIG. 12. Two ontogenetic stages of the primary zooecium of Bugitla calathns (after VIGELIUS). 

 A, invagination of the retractile disc to form the rudiment of the polypide (a); ft, cells of 

 the external layer of the polypide ; e, epithelium of ectocyst. B, a somewhat older stage. 

 The invagination forming the rudiment of the polypide has closed. In A the degenerating 

 mass derived from the larval organs is omitted. 



organs, and remain to form the mesodermal layer of the polypide. 

 These statements have not been confirmed by later researches. Far 

 more probable is the conjecture of OSTROUMOFF (Nos. 25 and 26) and 

 VIGELIUS (No. 40), which has recently been confirmed by PROUHO 

 (No. 28), that this cell-layer is derived from the mesodermal larval 

 tissue (central tissue). Of the remainder of this tissue, part seems to 

 yield the somatic mesodermal layer (the so-called parenchymatous 

 tissue), while another part joins with the granular masses which have 

 arisen through degeneration in the formation of the so-called brown 

 body. This body becomes connected with the stomach of the newly- 

 formed polypide and is finally absorbed. These processes will be 



