TROPHOBLAST AND SEROSA. 591 



whole. For the vertebrates^ Hubrecht has provided the requi- 

 site theory. The present paper is written for the purpose of 

 providing an analogous theory for the insects, while the 

 scorpions must be handed over to posterity. 



My chief object is to demonstrate tlie applicability of the 

 conception of the trophoblast to invertebrate animals ; in fact, 

 to show that the serosa of the insect embryo can be traced 

 back to a primitive trophoblast. 



I believe, in short, that the trophoblast, as it is pre- 

 served to us in the embryos of Peri patus novae-brit- 

 anniae, arose in adaptation to a viviparous habit 

 acquired by the terrestrial descendant of an aquatic 

 ancestor; and that it became transformed, whether 

 directly or by substitution, into the serosa, in corre- 

 lation with the secondary deposition of yolk-laden 

 eggs. 



In the case of the vertebrates, Hubrecht starts with a pro- 

 tective layer (Deckschicht), which becomes transformed into a 

 nutritive layer (trophoblast). For the insects, I commence with 

 a nutritive layer, which becomes changed into a merely pro- 

 tective layer, the serosa. This example will suffice to show 

 that the two theories are quite distinct in their treatment of 

 the respective problems, only they have the principle in common 

 which is expressed in Hubrecht's admirable conception of the 

 trophoblast. 



Just as, throughout the series of the Amniota, the forma- 

 tion of the amnion by no means takes place according to the 

 stereotyped plan with which we are familiar in the chick, so in 

 insects there is a distinct gradation in the mode and 

 extent of development of the amnion. It attains its 

 completest development in the highest order of insects, namely 

 the Hymenoptera, while its most nascent condition is exhibited 

 in the most primitive insect in which an amnion has been 

 found to occur, namely the Thysanurid, Lepisma saccha- 

 rina, L, (Heymons, 4). The observations of Heymons on the 

 development of Lepisma are particularly noteworthy. Not 



