CHAPTER VI. 

 GANOIDEI 1 . 



IT is only within quite recent times that any investigations 

 have been made on the embryology of this heterogeneous, but 

 primitive group of fishes. Much still remains to be done, but we 

 now know the main outlines of the development of Acipenser 

 and Lepidosteus, which are representatives of the two important 

 sub-divisions of the Ganoids. Both types have a complete seg- 

 mentation, but Lepidosteus presents in its development some 

 striking approximations to the Teleostei. I have placed at the 

 end of the chapter a few remarks with reference to the affinities 

 indicated by the embryology. 



ACIPENSER 2 . 



The freshly laid ovum is 2 mm. in diameter and is invested 

 by a two-layered shell, covered by a cellular layer derived from 

 the follicle 3 . The segmentation, though complete, approaches 



1 The following classification of the Ganoidei is employed in the present chapter : 



Q , . . jAcipenseridse. ( Polypteridse. 



lei< ^Polyodontid*. II. Teleostoidei. JAmiids. 



' Lepidosteidas. 



2 Our knowledge of the development of Acipenser is in the main derived from 

 Salensky's valuable observations. His full memoir is unfortunately published in 

 Russian, and I have been obliged to satisfy myself with the abstract (No. 90), and 

 with what could be gathered from his plates. Prof. Salensky very kindly supplied me 

 with some embryos ; and I have therefore been able to some extent to work over the 

 subject myself. This is more especially true for the stages after hatching. The 

 embryos of the earlier stages were not sufficiently well preserved for me to observe 

 more than the external features and a few points with reference to the formation of the 

 layers. 



3 Seven micropylar apertures, six of which form a circle round the seventh, are 

 stated by Kowalevsky, Wagner, and Owsjannikoff (No. 89) to be present at one of 

 the poles of the inner egg membrane. They are stated by Salensky to vary in number 

 from five to thirteen. 



