112 THE DEVELOPMENT OF 



The relations of the mesoderm to the proctodaeum would 

 also seem confirmative of this view, for, as will appear 

 later, most of it remains in the regioii where the anas is 

 formed. The mouth appears to arise scme distance in ad- 

 vance of the blastoporal region. 



While the phenomena of gastrulation are well developed 

 in most of the Crustacea, in the Hexapods and Acerata 

 (Arachnids jplus Limulus, Kingsley '85), they are so ob- 

 seured as to have caused no little trouble for students. It 

 seems to me that the facts detailed above for Crangon 

 throw some light upon the other members of the group and 

 show that the peculiar manner of origin of endoderm in 

 the old group of ^ Tracheates' is to be reconciled with the 

 gastrsea theory. 



The great majority of the arthropods have a segmenta- 

 tion which is usually characterized as superficial (Haeckel, 

 '75), or centrolecithal (Balfour, '80), both terms indica- 

 ting that the segmentation is confined to the surface of the 

 Qgg, while the centre is occupied by yolk which may, but 

 which usually does not, segment. The term endolecithal, 

 introduced by Claus, is synonymous with the earlier one of 

 Balfour. Bruce ('86) is, as far as I am aware, the only 

 one who has questioned this centrolecithal or superficial 

 terminology. He says that the process in Thyridopteryx 

 "can hardly be called a centrolecithal segmentation." 



These terms (centrolecithal, endolecithal, superficial 

 segmentation) seem unfortunate, for while there is a con- 

 siderable similarity in the mode of segmentation of most 

 arthropod eggs, in the earlier stagesthe yolk does not oc- 

 cupy a central position, nor is the segmentation superficial. 

 As I have shown above, the Qgg nucleus, and presumably 

 the segmentation nucleus, occupies, at first, at least in the 

 (ig^^ of Crangon, a central position ; while, gathered around 



