176 F. X. WILLIAMS 



rapidly absorbed, and, with the exception of a comparatively 

 small area at the cephalic end, is interposed between the amnion 

 and the serosa. The central yolk-mass, Y, is becoming envel- 

 oped by a thin entodermal sheet ENT. The latter encloses a 

 muscular layer on the ventral side of the yolk. The stomodaeum 

 and the proctodaeum (ST and PR, fig. 52), are thick- walled 

 tubes of columnar cells which are bounded externally by a mus- 

 cular layer. A shelf-like extension near the inner end of the 

 proctodaeum evidently represents a valve. A thin sheet of ento- 

 derm shuts off these two invaginations from the forming mid- 

 gut. The ectoderm, in general quite thick, thins out interseg- 

 mentally and where it passes over into the amnion. The pleuro- 

 podia, IP, have reached their full size, and have evidently dis- 

 charged most of the secretion. The central nervous system is 

 compact and proportionally of great size so that the connectives 

 are very short. The punktsubstanz or central fibrous mate- 

 rial, F, occupies the more dorsal inner part of the chain and is 

 enveloped by ectodermal epithelium, the inner neurilemma IN, 

 (innere Nervenscheide, neuroglia), while the chain itself is en- 

 sheathed by a thinner though more distinct layer, the outer 

 neurilemma or perineurium ON, also probably ectodermal. Both 

 of these envelopes are figured by Graber ('89) in Stenobothrus, 

 and by Wheeler ('89) in Doryphora. The remainder of the body 

 cavity is to a great extent, occupied by irregular sheets of fairly 

 well differentiated adipose tissue, by blood-cells, and by the 

 large rounded yellow pericardial cells. The gonads, GN, a com- 

 pact group of large cells, lie in the dorsal part of the fourth 

 abdominal segment. What I take to be the 'suboesophageal 

 body' of Wheeler is represented by a conspicuous bunch of 

 large yellowish cells, SG, which partly invests the posterior end 

 of the stomodaeum, on the ventral side. Except for the more 

 vacuolated and granular contents, these cells, closely resemble 

 the pericardial cells. Of the suboesophageal body, Wheeler ('92) 

 says in part: 



It is apparently the earliest organ to be differentiated from the walls 

 of the coelomic sacs. Its cells, at first wedge-shaped, gradually increase 

 in size, become rounded and highly vacuolated and resemble the fat- 



