1916.] NATURAL SCIENCES OF PHILADELPHIA. \S~> 



the connectives may continue to contain cells until a relatively late 

 stage in the development of the embryo. 



After the cerebral ganglia have developed, yet before they havi 

 become fashioned into their final form, the pleural ganglia and the 

 elements of the visceral loop put in their final appearance. Each 

 pleural ganglion is the product of cells migrating from the adjacent 

 ectoderm and arises at the sides of the body immediately posterior 

 and slightly dorsal to the cerebral ganglion. In most instances it is 

 a fairly sharply defined though sheet-like structure, yet it may 

 originate in the form of several distinct masses whose anastomosing 

 fibres form a distinct plexus before they become fused into a single 

 mass. 



The ganglia of the visceral loop arise simultaneously. The sub- 

 intestinal occupies a position on the right side of the body at the 

 intersection of the neck and visceral mass; the supra-intestinal lies 

 approximately in the midline at the junction of the neck and visceral 

 mass; while the visceral ganglion is somewhat to the right of the 

 midline and anterior to the terminal section of the intestine. Even 

 while these ganglia are in process of formation delicate fibrils from 

 some of the component, bipolar cells push out between the yolk 

 granules and increasing in number ultimately form the connectives 

 of the visceral loop. The osphradial ganglion rapidly becomes 

 imbedded in the margin of the mantle, and, as in the adult, is united 

 by a connective with the supra-intestinal ganglion. In many 

 instances a delicate connective unites the sub- and supra-intestinal 

 ganglia; no trace of it has been found to exist in adult specimens. 

 The right-sided zygoneurous condition, the union of the pleural and 

 the sub-intestinal ganglion, characteristic of the adult appears to put 

 in an appearance shortly before the embryo escapes from the parent. 



At the time of hatching there are traces of a plexus extending 

 about the margin of the mantle, but it is very dim and indistinct. 

 When it becomes clearly defined throughout and capable of being 

 traced it has all of the essential features of the marginal mantle 

 nerve of the adult. 



With the increased growth of the mantle and the development of 

 an extensive mantle cavity (beyond the stage represented in text 

 fig. 2) the visceral mass between the mantle margin and the neck 

 becomes completely covered. Furthermore, the absorption of the 

 included yolk creates a more flattened condition of the embryo, 

 which ultimately brings the nervous system into the position charac- 

 teristic of the adult. 



