[43] EMBRYOGRAPHY OF OSSEOUS FISHES. 49 7 



development advances the medullary groove becomes less and less 

 marked in depth, and by the time the embryo's body has been fairly 

 outlined there is nothing more of it visible in sections. For my part 

 I now have serious doubts as to whether any actual infolding of the 

 sensory layer of the blastoderm of Teleosts ever takes place to form 

 the ueurula. It would almost seem as if the formation of the medul- 

 lary plate took place rather by the slow heaping up of the cells of the 

 sensory layer along the neural axis by an amoeboid or migratory pro- 

 cess. It is at any rate difficult, if not impossible, to find the evidence 

 of any process of infolding of the nervous layer to form the neurula 

 such as has been observed in other types. The mode of development of 

 the neurula, however, as it maybe observed in Teleosteans, Amphibians, 

 Elasmobranchs, Marsipobranchs, AmpMoxus, birds, and mammals, dif- 

 fers so widely in detail and essentials in these different groups as to 

 hinder us from framing any general theory of development for the 

 nervous system of the vertebrates. Such a procedure is all the more 

 to be regarded as premature, in view of the fact that jve do not yet 

 know the full history of the development of such forms as Myxine, 

 Lepidosteus, and Amia. What there may still remain to be revealed 

 of a startling or unexpected character, in a study of these forms, we do 

 not know, for the development of none of them is thoroughly known. 

 In fact, the development of comparatively few animals is as thoroughly 

 known for all of their stages as will be demanded by the comjiarative 

 embryology of the future, so ably heralded by the late Dr. Balfour. 

 The science in its present state may, on account of the imperfection of 

 most of the developmental histories of the principal types, be com- 

 pared to an ancient manuscript of which just enough has been pre- 

 served to give us an idea of the way it treats its subject. Great gaps 

 in our embryological knowledge are apparent in even some of the best 

 studied forms. In one form we know the early history; in another the 

 later. In others we know the development of the germs in the repro- 

 ductive organs before impregnation ; in others we do not. In some 

 cases we know the late phases of development when the embryo or 

 young nasses into the adult condition ; in others our knowledge in this 

 respect is a blank. Not only is this a serious difficulty, but there is 

 also the still more serious one of reconciling the contradictory state- 

 ments and observations of honest investigators each of whom has usu- 

 ally added some important information to that which we previously 

 possessed, but who have rarely missed falling into errors of interj)re- 

 tation due to the nature of the subject, defective opportunities and 

 methods; or on account of the finite nature of the mind itself they 

 have been more or less mistaken in making inferences and deductions 

 from the observed facts. This is no discredit to the science, but only 

 a necessary condition through which it must pass in the course of its 

 development. 



The blastoderm of the cod's egg, like that of other Teleosts, continues 

 S. Mis. 46 32 



