OF OECANTHUS AND TELEAS. 



259 



egg, and by a subsequent growth entirely surrounds the endodcrmic sac and its contents. 

 This process is modified, however, by the interpolation of the so-called embryonic membranes. 

 Since the endodermic sac is so extremely large — as compared with the size of the embryo — 

 that the latter could not easily inclose it by a simple epiboly much before the time of 

 hatching, it is apparent that any modification of the primitive process which will enable 

 the embryo to enclose the yolk earlier will be advantageous to the animal by shortening 

 the developmental processes. Hence, the lateral edges of the band-like embryo instead of 

 epibolizing the yolk have acquired through adaptation the tendency to grow ventrad 

 instead of dorsad, so that on the completion of the amnion, the embryo is developed inside 

 out, with its dorsum (the amnion) covering the ventral surface of the germinal band after 

 the fashion of a membrane. 



That the cavity inclosed between the so-called amnion and the germinal band is really 

 external to the embryo is evident from the fact that the external ends of the stomodaeum 

 and proctodaeum communicate with it. The fusion of the two edges of this amnion in the 

 median ventral line, may take place simultaneously with that of the serosa, or it may take 

 place quite independently of the serosa and at a later period. The embryo may, or may 

 not, become entirely disunited from the endodermic sac (now serosa), and hence may be 

 related to it in one of three ways. It may be in contact with, it may be fused with, or it may 

 lie entirely within, the serosal sac. This process is capable of being explained in two ways. 

 Either the yolk mass would not allow the dorsad growth of the lateral edges of the germinal 

 band, or it is strictly an adaptive response of the embryo to changed conditions of environ- 

 ment. The latter is probably the true cause of the unique phenomenon of an animal under- 

 going development inside out, and its object can only be the swallowing of the food stored 

 up for its nourishment. The amnion and serosa fuse in the head region of the embryo. This 



fused membrane soon ruptures, so that the sac-like embryo, which 

 up to this has had no structural connection with or control 

 over its food sac, becomes united with the serosa into a double 

 sac, a sac within a sac'. 



The embryo now everts itself through this opening and lies 

 outside of the yolk sac, which in the meantime has assumed a 

 position dorsad to the embryo: The amnion now forms the dor- 

 sal and part of the pleural walls of the embryo and at its ceph- 

 alic termination is continuous with the yolk sac. The yolk to- 

 gether with the greater part of the cell elements forming the wall 

 of the yolk sac is soon enclosed within the body of the embryo. 

 The remnant of the yolk sac remains for some time as a plug- 

 like projection from the median dorsal line immediately behind 

 the head, and is the homologue of Kowalevski's dorsal organ. 



Since the appearance of Kowalevski's paper on the embryonic 

 development of Hydiophilus, in which he first makes known the 

 existence of a dorsal organ among insects, the conjectures 

 as to its probable significance have been various. The follow- 

 ing explanation of its origin and function is a necessary corollary 

 from the previous explanation of the embryonic membranes. 



Fig. 29. 



Diagrammatic representation of the 

 relations of the mesenteron, stomo- 

 daeum, find proctodaeum after the 

 closure of the body walls over the 

 dorsal organ or plug. Compiled from 

 sections of an emurvo. X50. 



