EMBRYOLOGY 



* 

 I 



anterior part, which remains open). The more internal layer of 

 the periphery of this closed canal is the origin of the endo- 

 derm and its derivatives. Subsequently the ventral plate and 

 its derivatives grow so as to form the ventral part and 

 the internal organs of the Insect, the dorsal part being com- 

 pleted much later by growths that differ much in different 

 Insects ; Graber, who has specially investigated this matter, 

 informing us ^ that an astonishing multifariousness is displayed. 

 It would appear that the various modes of this development 

 do not coincide with the divisions into Orders and Families 

 adopted by any systematists. 



We should observe that the terms ectoderm, mesoderm, and 

 endoderm will probably be no longer applied to the layers of the 

 embryo when embryologists shall have decided as to the nature 

 of the derived layers, and shall have agreed as to names for 

 them. According to the nomenclature of Graber^ the blasto- 

 derm differentiates into Ectoblast and Endoblast ; this latter 

 undergoing a further differentiation into Coeloblast and Myoblast. 

 This talented embryologist gives the following table of the 

 relations of the embryonic layers and their nomenclature, the 

 first term of each group being the one he proposed to use : 



Protoblast.' 



'Periblast 

 (Epiblast, 

 blastoderm). 



^Centroblast 

 (Yolk-cells, hypo- 

 blast, endoderm 

 in part of Bal- 

 four). 



-Ectoblast 

 (Ectoderm, outer 

 layer). 



^Endoblast 



or Hypoblast^ 



inner layer. 

 (Mesoblast of Bal 



four.) 

 Mesoderm and endo 



derm. 



Part of yolk cells. 



Coeloblast 

 (Endoderm in nar- 

 rower sense). 

 Darmdriisenblatt. 



Myoblast (Meso- 

 derm of most 

 authors). D:trm- 

 niuskelblatt. 



Nussbaum considers ^ that " there are four layers in the cock- 

 roach-embryo, viz. (1) epiblast, from which the integument and 

 nervous system are developed ; (2) somatic layer of mesoblast, 

 mainly converted into the muscles of the body- wall; (3) splaiichnic 

 layer of mesoblast, yielding the muscular coat of the alimentary 

 canal; and (4) hypoblast, yielding the epithelium of the mesen- 

 teron." 



Turning our attention to the origin of the segmentation, that 

 is so marked a feature of Insect structure, we find that evidence 



1 DcnTc. AJc. TFicn, Iv. 1888, p. 109, etc. ^ Morph. Jahrb. xiv. 1888, p. 347. 



3 In Miall and Denny, Cockroach, p. 188. 



