48 DEAN. [Vol. XI, 



IV. General Comparison of the Early Stages of Gar- 

 pike AND Sturgeon, and Conclusions. 



In the foregoing paper the similarity in the mode of develop- 

 ment of these kindred Ganoids has been noted in some detail 

 from stage to stage. There have thus been compared their 

 rate of development (p. 12), the positions assumed during the 

 growth of the embryo (p. 13), the size, adhesiveness, mode of 

 pigmentation and deposition of the ^-^^ (p. 14), the cleavage 

 (pp. 30-34), gastrulation (p. 39), and the origin of the meso- 

 derm (p. 47). There yet remains to be given a summary of the 

 results of the present writer which shall contrast the general 

 processes of growth, z>., segmentation, the development of the 

 primary germ layers and the establishment of the embryo's 

 external form. 



The segmentation of Lepidosteus appears clearly of a more 

 meroblastic character than hitherto described : in none of the 

 writer's material have cleavage planes been observed travers- 

 ing even superficially the yolk pole of the Q-ZZ- The germ 

 substance is sharply separate from the yolk ; its specific 

 gravity appears notably lighter, and at the animal pole it often 

 projects above the surface curvature of the yolk mass. Cleav- 

 age is early expressed in regular planes, giving the blastomeres 

 almost conventional outlines ; it is only after the fourth cleavage 

 that variations are observed. 



In Acipenser, on the other hand, segmentation is holoblastic, 

 although the furrows traversing the yolk region are of an 

 exceedingly superficial character.^ The consistency of the 

 substance of the germ is more nearly that of the yolk, and the 

 surface curvature of the early blastomeres corresponds with 

 that of the Q.gg. In general cleavage planes are deepest in the 

 region of the animal pole, becoming more and more superficial 

 as they pass around the Q.g^ to its lowermost point : they agree 

 in general direction with those of Lepidosteus but are irregular 

 in their mode of occurrence from the second plane onward. 



1 As noted by Salensky and others. 



