CHAPTER VI. 



GANOIDEI'. 



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 segmentation, but Lepidos- 

 teus 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. 



AcipenserI 



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^. 

 The segmentation, though complete, approaches the meroblastic type 

 more nearly than the segmentation of the frog's egg. The first furrow 

 appears at the formative pole, at which the germinal vesicle was 

 situated. The earlier phases of the segmentation are like those of 

 meroblastic ova, in that the furrows only penetrate for a certain 

 distance into the egg. Eight vertical furrows appear before the first 

 equatorial furrow; which is somewhat irregular, and situated close to 

 the formative pole. 



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

 I Splarhoirlpi iA-cipenseridae. ( Polypterida3. 



1. aeiacnoiaei. \Poiyodontida). ii. Teleostoidei. JAmiidiB. 



( Lepidosteidae. 



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

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

 Kussian, 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. 



2 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. J 



