hORMATJON OF MEMBRANES AND PRIMITIVE HANI). 241 



is by no means constant in Insects. I shall hereafter show that 

 it is an extreme modification of the usual process. It has been 

 described by Graber, who speaks of it as the ventral ptycho- 

 blast, and he believes that the mesoblast is formed from it. 

 The mesoblast in the fly is not so developed, and I think there 

 can be little doubt that Graber's ventral ptychoblast is in 

 reality the amnion, and has nothing whatever to do with the 

 median groove in the primitive band, and this groove has 

 nothing whatever to do with the formation of the archenteron ; 

 therefore whatever view is taken, it cannot be gastrulation. 



The very prevalent idea that the median groove in the primi- 

 tive band of insects represents a blastopore, which originated 

 with Sedgwick, and was supported by Balfour, is very ably con- 

 troverted by Kennel [104]. It appears to me to be entirely due 

 to a misconception. It is of course absolutely untenable if my 

 conclusions and observations are correct. 



2. FORMATION OF THE MEMBRANES AND PRIMITIVE 

 BAND. 



The Embryonic Membranes of insects are an e.xternal mem- 

 brane, termed the serosa, and an internal, the amnion. Similar 

 membranes are also found in other Arthropods, but authors are 

 by no means agreed as to the real nature and homologies of 

 these. In many insects the greater part of the cellular layer 

 enclosing the yelk, the blastula, becomes converted into the 

 membranous coverings of the embryo. 



Relation of the Primitive Band to the Amnion and Serosa. — In 

 most insects the primitive band (see p. ij) and the rudi- 

 mentary procephalic lobes appear upon the ventral aspect of the 

 blastoderm before there is any amnion or serosa developed, and 

 the latter are formed either by the sinking of the embryo into the 

 } elk, as in the Lepidoptera, or by the growth of a fold over the 

 ventral surface of the embryo from its edges. In both cases 

 the e.xternal layer is termed the serosa, and the internal the 

 amnion. In some insects, as in the Lepidoptera, the yelk 

 passes into the inter-amnial space ; in others the yelk is 

 entirely on the dorsal surface of the embryo. 



