436 Avery E. L;uiil)ert. 



inal segments, from which the nen^e cord is derived. Thus the cere- 

 bral ganglia are seen to maintain the same relation to the pre- 

 cheliceral segments that the neuromeres which form the rudiment of 

 the nerve cord hold to the thoracic and abdominal segments, and 

 may be regarded as being serially homologous with them. 



The first, second, and third optic ganglia now appear as thickened 

 areas which lie between the cerebral ganglia and the outer margins 

 of the lobes. The optic plate ( o. pi-), or thickened margin of the 

 optic gauglia, consists of a continuous band of thickened ectoderm 

 bordering the lateral edges of the cephalic plate. 



Stage V. Fig. 34- — The dorsal flexure reaches its extreme limit 

 during the preceding stage. After this period a remarkable change 

 occurs, and, instead of the cephalic and caudal ends of the embryo 

 approaching one another on the dorsal side, the embryo is gradually 

 bent in the other direction so that the two ends approach each other 

 on the ventral surface. 



According to Balfour and Locy, with whom Kishinouye is in sub- 

 stantial agreement, this change of flexure from the dorsal to the 

 ventral direction is due to the increasing growth of the dorsal surface 

 and the shortening of the germ bands. Both of the authors referred 

 to fail to mention what is, in, their opinion, the cause of this shorten- 

 ing of the lateral plates of the embryo. 



Morin maintains that the change is due to the shifting of the yolk 

 from the ventral to a more dorsal position. But it is to be observed 

 in Epeira that the yolk never bulges ventrally in a more prominent 

 manner than during the first stages of the reversion period, a condi- 

 tion which the greater extension of the lateral plates over the dorsal 

 surface of the egg is well calculated to produce. 



It seems probable that Balfour and Locy have given a correct 

 answer to this problem in so far as Ave are to look for the cause of 

 this change, not in the shifting of the yolk, but in the growth of the 

 embryo itself. Since the position of the yolk is purely relative, any 

 change in the growth of the lateral plates would result in a change 

 in its position. The growth of the lateral plates continues dorsal- 

 wards until they meet in the mid-dorsnl line Avhere they enclose a 

 portion of the original coelom which becomes the cavity of the heart. 



