54 EMBEYOLOGY OF THE LOWER VEETEBEATES ch. 



Eeptiles, and that the neurenteric canal represents a persisting 

 portion of a once slit -like gastrula mouth which is otherwise 

 obliterated. 



ORIGIN OF THE MESODERM 



General Remabks. — Already during the process of segmentation 

 the differentiation of the two primary cell-layers commences — the 

 superficial cells towards the apical pole dividing more actively, being 

 smaller, and being less laden with food-yolk and thus establishing 

 a character of their own as ectodermal cells. The full establishment 

 of the primary layers however is only consummated during the 

 process of gastrulation when the ectoderm comes by the various 

 processes already described to enclose the remaining cells the 

 (archenteric) endoderm. 



The establishment of the two primary layers is followed 

 immediately (indeed the two processes frequently overlap) by the 

 development of the intermediate cell-layer — the mesoderm — which 

 will in the adult form the great mass of the body — all in fact except 

 the epidermis and its derivatives on the one hand and the enteric 

 epithelium and its derivatives on the other. 



The problem of the evolutionary history of the mesoderm of 

 Vertebrates is one upon which there is little agreement. Anything 

 of the nature of elaborate and detailed treatment of the subject 

 would be out of place in a textbook of moderate size and a short 

 sketch such as the following is necessarily coloured by the general 

 morphological views of the writer. While the views set forth in the 

 following paragraphs seem to the author to fit most satisfactorily the 

 facts so far as these are established beyond reasonable doubt there 

 are other embryologists who would give an account differing consider- 

 ably from that given here. 



To the present writer it seems of importance in endeavouring to 

 arrive at reliable general conclusions from the facts of observation to 

 bear in mind particularly the risk of reaching erroneous conclusions 

 through basing arguments upon phenomena observed in the head 

 region or tail region of the embryo. Intense cephalization, i.e. 

 intense structural modification of the anterior region of the body, to 

 form a head, is admittedly one of the fundamental characters of the 

 phylum Vertebrata. In this modification the mesoderm has been 

 deeply involved so that there is always a considerable weight of 

 probability against conditions observed in the head region being 

 primitive. Again the tail region is also intensely modified, as is 

 indicated e.g. by the transient appearance within it of a vestigial 

 portion of alimentary canal with surrounding body-cavity. Here 

 again then, though not to the same extent as in the head-region, 

 suspicion rests upon the primitiveness of all phenomena of develop- 

 ment peculiar to this region of the body. 



It is advisable then, for these reasons, to exercise great caution in 



