DEVELOPMENT OF ELASMOBRANX'H FISHES. G7 



ably also present in the blastoderm of most Osseous Fishes, 

 though not noticed by Professor Haeckel in the investigations 

 recorded in his paper. 



The facts of development of Osseous Fishes, upon which Pro- 

 fessor Haeckel rests his views, are too much disputed, for their 

 discussion in this place to be profitable \ The eggs of Osseous 

 Fishes appear to me unsatisfactory objects for the study of this 

 question, partly on account of all the cells of the blastoderm 

 being so much alike, that it is a very difficult matter to 

 distinguish between the various layers, and, partly, because 

 there can be little question that the eggs of existing Osseous 

 Fishes are very much modified, throuc^^h havino^ lost a o-reat 

 part of the food-yolk possessed by the eggs of their ancestors". 

 This disappearance of the food-yolk must, without doubt, 

 have produced important changes in development, which would 

 be especially marked in a pelagic egg, like that investigated 

 by Professor Haeckel. 



The Avian egg has been a still more disputed object than 

 even the egg of the Osseous Fishes. The results of my own 

 investigations on this subject do not accord with those of Dr 

 Gotte, or the views of Professor HaeckeP. 



1 A short statement by Kowalevskv on this subject in a note to his account of 

 the development of Ascidians, would seem to indicate that the type of development 

 of Osseous Fishes is precisely the same as that of Elasmobranchs. Kowalevsky 

 says, Arch. f. Micr. Anat. Vol. vii. p. 114, note 5. " Accordmg to my observa- 

 tions on Osseous Fishes the germinal wall consists of two layers, an upper and 

 lower, which are continuous with one another at the border. From the upper 

 one develops skin and nervous system, from the lower hypoblast and mesoblast." 

 This statement, which leaves unanswered a number of important questions, is 

 too short to serve as a basis for supporting my views, but so far as it goes its 

 agreement with the facts of Elasmobranch development is undoubtedly striking. 



2 The eggs of the Osseous Fishes have, I beheve, undergone changes of the 

 same character, but not to the same extent, as those of Mammaha, which, 

 according to the views expressed both by Professor Haeckel and myself, are 

 degenerated from an ovum with a large food-yolk. The grounds on which I 

 regard the eggs of Osseous Fishes as having undergone an analogous change, are 

 too foreign to the subject to be stated here. 



3 I find myself imable without figures to understand Dr Eauber's {Central- 

 blatt fiir 2Ied. Wiss. 1874, No. 50; 1875, Nos. 4 and 17) views with sufficient 

 precision to accord to them either my assent or dissent. It is quite in accord- 

 ance with the view propounded in my paper {loc. cit.) to regard, with Dr Eauber 

 and Professor Haeckel, the thickened edge of the blastoderm as the homologue 

 of the lip of the blastopore in Amphioxus ; though an invagination, in the manner 

 imagined by Professor Haeckel, is no necessary consequence of this view. If Dr 

 Eauber regards the whole egg of the bird as the homologue of that of Amphioxus, 

 and the inclosure of the yolk by the blastoderm as the equivalent to the process 

 of invagination in Amphioxus, then his views are practically in accordance with 

 mv own. 



