280 DEVELOPMENT OF ELASMOBRANCH FISHES. 



discussion in this place to be profitable 1 . 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 dis- 

 tinguish between the various layers, and, partly, because there 

 can be little question that the eggs of existing Osseous Fishes 

 are very much modified, through having lost a great part of the 

 food-yolk possessed by the eggs of their ancestors 2 . This dis- 

 appearance 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 Haeckel 3 . 



Apart from disputed points of development, it appears to me 

 that a comparative account of the development of the meroblastic 



1 A short statement by Kowalevsky 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. Mikr. Anat. Vol. vn. p. 114, note 5, "According to my observations 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. 



: The eggs of the Osseous Fishes have, I believe, undergone changes of the same 

 character, but not to the same extent, as those of Mammalia, 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 unable without figures to understand Dr Rauber's ( Centra Iblatt 

 fitrMed. 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 accordance with the view 

 propounded in my paper (loc. cit.) to regard, with Dr Rauber 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 Rauber 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 my own. 



