No. I.] GAR-PIKE AND STURGEON. 49 



In pigmentation the eggs of Acipenser present the strongest 

 contrast to those of the Gar-pike. 



The stages of late segmentation may thus be contrasted. 

 In Lepidosteus the division of the blastomeres results in a cap 

 of cells of irregular sizes : this appears as a distinct prominence 

 of the egg surface : below it a smooth surfaced periblast-like 

 layer contributes at irregular points to the growth of the blasto- 

 derm, — a condition maintaining till the time of gastrulation. 

 The cell cap of Acipenser is far more closely continuous with 

 the yolk ; cells large and irregular in size are readily dis- 

 tinguished in an intermediate zone between blastoderm and 

 yolk : of this zone the number of tiers of cells agrees in a 

 general way with that of the merocytes in corresponding stages 

 of Lepidosteus. 



The general differences in segmentation appear due to either 

 a greater or decreased amount of the egg's reserve of yolk 

 material. But, everything considered, it seems to the present 

 writer more probable that the mode of segmentation of Acipen- 

 ser, although holoblastic, is more readily to be derived from that 

 of Lepidosteus than vice versa. The reasons for this view of the 

 relationship of the Sturgeon are based : (i) on the knowledge 

 of its descent as afforded by palaeontology,^ (2) on the curiously 

 superficial character of its total segmentation, (3) on its marked 

 irregularity of cleavage, — a character by no means conclusive 

 in itself, but important in connection with (i) and (2). If the 

 evidence be ultimately accepted as to the derivation of the 

 Sturgeon and of its mode of segmentation, it is obvious that 

 the results of Beard in his paper on the Interrelationships of 

 the Ichthyopsida would be seriously disarranged. 



In the development of the primary germ layers the differ- 

 ences of these forms appear in like manner due to the altera- 

 tion in quantity of yolk-material. In Lepidosteus the clearly 

 marked surface between cell cap and yolk maintains until the 

 time of gastrulation ; in Acipenser a transitional zone of irregu- 

 lar yolk cells exists from an early stage. In one form the cap 

 of cells is of irregular character, in the other it is reduced to a 



1 Cf. esp. A. Smith Woodward, On the Palaeontology of Sturgeons, Pro. Geol. 

 Ass., Vol. IX, Nos. I and 2. 



