EMBRYONIC ENVELOPES, DORSAL ORGANS, BLASTOKINESIS 51 



The qviestion as to the homologies of the amnion and serosa witli 

 reference to structures existing in the Arthropoda lacking these envelopes 

 has frequently been raised by embryologists. With the Collembola there 

 is no unanimity of opinion. Heymons thought that the primary dorsal 

 organ of the Onychophora, Myriapoda, and Apterygota was a structure 

 homologous with the serosa of the Pterygota; but as Philiptschenko (1912) 

 has pointed out, this is impossible, since in the beetle Donacia there is a 

 serosa as well as a rudimentary primary dorsal organ. Willey (1899) 

 regarded the dorsal organ of the Onychophora 

 and of the Collembola as being homologous 

 with the indusium, a structure exceptionally 

 found among pterygotes, and believed that 

 the serosa and the dorsal organ among 



these arose independently. Hirschler (1909) Hi "K" '^SDCh ^^^i — "^9^ 

 found a cell mass appearing early in the 

 embryo of Donacia which he identified with 

 the dorsal organ in Isotoma. Philiptschenko 

 (1912) held that the envelope-like extra- 

 embryonic part of the primary epithelium Fig. 32. -Provisional closure 



(blastoderm) in Isotoma, which we shall call of dorsal wall by the amnion 

 the "amnioserosa," is the homologue of the 'S' ,.f rVnfS.etbl: 

 combined amnion and serosa of the ptery- secondary dorsal organ {do). 

 gotes. Finally, Strindberg (1913b) suggested <^',f «^^!™- „S|Se?r"°°- 

 the use of the term "proserosa" for the dorsal 



organ in Isotoma and "proamnion" for the extraembryonic lateral walls, 

 thus indicating his opinion of the homologies. 



Heymons' interpretation of the homologies of the amnion, serosa, and 

 secondary dorsal organ among the Onychophora, Myriapoda, Thysanura, 

 and Pterygota, however, seems to rest upon a firmer basis. He points 

 to the occurrence in the neck region of the embryo of Peripatus capensis 

 of a thickened area of vacuolated cells which Sedgwick designated as the 

 "ectodermal hump" and which corresponds in position to the secondary 

 dorsal organ of the Pterygota. It is doubtless the same type of structure 

 that Willey (1899) described as the trophic vesicle in P. novae-hritanniae 

 and that corresponds both as to position and fate to the secondary dorsal 

 organ of the higher insects. In Scolopendra the elongated germ band 

 of an intermediate stage embryo occupies the ventral side, the memhrana 

 ventralis (vm), the dorsal side of the egg. Immediately behind the head 

 there is a small disk, which, as development proceeds, thickens and 

 becomes several-layered. Later the lower layers of the disk degenerate 

 in the yolk while the surface layer, together with the memhrana dorsalis, 

 forms the definitive dorsal closure of the animal. Comparing the stage 

 just described with that of a ptery gote insect that has just undergone 



