CHAPTER XI 



EMBRYO FORMATION IN OTHER GROUPS OF 

 VERTEBRATES 



UNTIL fairly recently embryology has been in the main a comparative study. 

 Its object has been conceived to be the derivation of a general scheme 

 of development, of which the processes found in the different groups of 

 animals could be regarded as modifications. In particular the diverse, but 

 nevertheless obviously related, groups of vertebrates have provided 

 fascinating material for study of the relations between different types of 

 embryonic development. The wealth of available facts is indeed immense, 

 and there are numerous textbooks devoted to it. Perhaps the best are 

 Dalcq and Gerard's revision of Brachet's work and the recent book of 

 Nelsen. 



Comparative embryology of what may be called the classical kind was 

 historically a derivative of, and theoretically should include, comparative 

 anatomy. The comparisons instituted were between particular stages of 

 the embryos of the various groups, each stage being considered as a static 

 anatomical form.The results of such study are of great interest in relation 

 to the general processes of evolution, though we consider them nowadays 

 rather as raising problems than contributing materially to their solution. 

 From the point of view on embryology which underlies the writing of 

 this book, this aspect of biology remains rather peripheral, and since an 

 adequate treatment of it requires the citation of an enormous number of 

 detailed facts, we shall have to omit it and leave it in the hands of the 

 comparative anatomists. The comparative method is still, however, a 

 valuable tool of analysis. It can be applied not only to anatomical data but 

 also to information of the kind with which this book has been mainly 

 concerned. Such applications have only recently been attempted, but there 

 are two types of comparative embryology which one must expect to 

 increase greatly in importance in the near future. One is the comparative 

 study of dynamic anatomical processes. Among the vertebrates there 

 is a body of material which most urgently calls for treatment in tliis way, 

 namely the active processes of gastrulation in the various groups which 

 have been revealed by the use of vital staining or marking. Secondly, we 

 now know enough about the causal sequences of epigenetic processes in 

 the early development of the various groups to begin to institute com- 

 parisons between them, and to enquire what kind of alterations in these 



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