DEVELOPMENT OE THE PHTLACTOL^MATOUS POLTZOA. 501 



lophophore, with tlie exception of the two opposed margins of the 

 arms, where they sprout first after the polypide can protrude itself 

 from its cystid. Nitsche compares the arms of the lophophore to 

 two great primary tentacles, from which the secondary (or defini- 

 tive) tentacles sprout. Prom this account of the orifice of the ten- 

 tacles it is obvious that their outer epithelium, which afterwards 

 becomes ciliated, is derived from the ectoderm of the cystid, which 

 corresponds to the inner layer of the bud ; while their inner epithe- 

 lium is derived from the endoderm of the cystid, the outer layer 

 of the bud. 



Already, long before these last-mentioned occurrences take 

 place, the nervous ganglion has made its appearance. At the 

 margin of the mouth, between the bases of the arms of the 

 lophophore, there may be seen an introversion of the two cellular 

 layers. This is so situated that its shallow lumen communicates 

 with the cavity of the tentacular sheath. Its direction is thus 

 the reverse of that of every introversion hitherto described as 

 occurring in the development of the bud ; for none of these open 

 into any part of the cavity of the bud, but, on the contrary, have 

 their lumen always in communication with the cavity of the 

 cystid. The margins of the shallow introversion thus formed now 

 grow together, in quite the same way as the margins of the me- 

 dullary groove in the vertebrate embryo. In this way a button- 

 like vesicle, composed of the two cellular layers of the bud, is 

 pushed out from the walls of that part of the bud which is to be- 

 come the pharynx of the polypid. It is the first sketch of the 

 nervous ganglion, and at this stage is relatively very large. Erom 

 its mode of formation, it is obvious that its internal proper nervous 

 substance proceeds from the internal layer of the bud, which is 

 derived from the ectoderm of the cystid ; while its envelope is a 

 continuation of the external layer of the bud, the endoderm of 

 the cystid. 



The phenomena here described are in accordance with the gene- 

 ral law that the central nervous system is always derived from 

 the ectoderm of the embryo ; but, as Nitsche suggests, we must 

 not in the present instance lose sight of the fact that the inner 

 layer of the bud, though arising from the ectoderm of the cystid, 

 has fundamentally different relations from those of an ordinary 

 ectoderm, for there proceeds from it, at the same time with the 

 nervous substance of the ganglion, the entire epithelial lining of 

 the intestinal tract. 



