50 DEAN. [Vol. XI. 



definite layer. In Gar-pike the segmentation cavity, when 

 definitely formed, undergoes the smaller changes in extent and 

 outline, and its roof remains of more uniform thickness. The 

 alterations which these conditions have been seen (p. 38) to 

 undergo in the Sturgeon seem of more specialized character. 

 Early gastrulation is closely comparable in both forms : that of 

 Lepidosteus, although hampered in development by the pres- 

 ence of a greater amount of yolk material, seems more general- 

 ized : in this form for example, as previously noted, p. 22, a 

 two-layered condition maintains until the time of the blasto- 

 pore's closure, the inner and outer layer of the dorsal lip are 

 essentially identical in structure, and as in the elasmobranchian 

 gastrula there occurs no growth modification of the rim of the 

 blastopore to give rise to a Kupffer's vesicle. It also ap- 

 proaches the type of the Elasmobranch in the character of the 

 superficial cell stratum, which it early develops, but diverges more 

 widely than Acipenser in the matter of neurenteric canal. Of 

 the development of the middle germ layer, p. 46, the conditions 

 that have been noted in Acipenser seem more specialized than 

 those of Lepidosteus ; thus the early peristomal mesoderm in 

 the latter form is independent in its distal margin from either 

 outer and inner layers, and presents a strong contrast to the 

 restricted and abbreviated layer of Sturgeon, The gastral 

 mesoderm, moreover, although similar in both forms in its 

 apparent mode of origin, suffers in Sturgeon a flattened growth 

 which can only be regarded as of specialized character. 



The growth processes establishing the outward form of the 

 embryo furnish perhaps the strongest ground of contrast in the 

 early development of Gar-pike and Sturgeon : in the former 

 the form outline of the embryo is constricted off from the yolk 

 mass ; in the latter the embryo, in a mode of growth greatly 

 flattened, continues up to a late stage to surround the yolk, and 

 to preserve outwardly the spherical curvature of the egg. 



The greatly flattened growth conditions of the Sturgeon 

 embryo are certainly unique among vertebrates ; and among 

 fishes — using the word in its broadest sense, — they may 

 reasonably be regarded as peculiarly aberrant. The mode of 

 establishment of the early external form of the Gar-pike is not 



