592 AETHUR WILLEY. 



only is the amniotic cavity of greater relative cubic capacity 

 in Lepisma than in any other Hexapod^ but it never com- 

 pletely closes^ remaining permanently open by a minute orifice, 

 the amniotic pore. Thus^ in its most primitive condi- 

 tion, the amniotic cavity of insects is an open sac. 

 This is in strong contrast with the vertebrate amniotic cavity, 

 as exemplified in the embryo of Erinaceus. Two quotations 

 from Hubrecht's memoir will suffice to call to mind the point 

 of view adopted by him with regard to the vertebrate amnion. 



He says (6, p. 24) : " Ich denke mir das Amnion in seiner 

 allerersten Entstehung gleich als geschlossene Blase ; ^ ich 

 betrachte somit die Entwickeluugsweise durch sich entgegen- 

 wachsende Faltenrander als ein bedeutend cenogenetisch modi- 

 ficirten Entwickelungsprozess.^' 



Again, on p. 25 he says : " Meiner Ansicht nach muss man 

 nicht bei Sauropsiden, sondern bei den Saugeru nach Andeu- 

 tungen der primitiveren, der mehr urspriinglichen Amnion- 

 entwickelung suchen." 



In Peripatus novse-britannise the egg is without yolk 

 and the embryo develops on the surface of a relatively enormous 

 blastodermic vesicle, whose wall consists of an internal layer of 

 endoderm and an external layer of ectoderm. The ectoderm 

 of this trophic vesicle does not consist of flattened and passive 

 cells, as does the serosa of insects, but it is a mucous epithelium, 

 the cells having vacuolar contents and serving for the absorp- 

 tion of nutrient fluids from the uterine wall and their trans- 

 ference to the trophic cavity, whence they are available for the 

 nutrition of the embryo. The trophic ectoderm is there- 

 fore the trophoblast of the embryos of this species of 

 Peripatus. The chorionic membrane always intervenes be- 

 tween the trophoblast and the uterine mucosa. 



At first the embryonic tract lies at the posterior ventral 

 extremity of the trophic or blastodermic vesicle, and at this 

 stage the entire embryo bear? a remarkable resemblance to an 



^ The posterior amuiotic canal of Ciielonians (Mitsukuri) and Hatteria 

 (Dendy) requires special explanation. 



