No. I.] GAR-PIKE AND STURGEON. 23 



irregular floor of loosely associated cells, (4) the even thickness, 

 homogeneity, and sharp demarcation of its germ disk (outer 

 germ layer), (5) the mode of its invagination, especially in the 

 mode of union of the invaginated layer with yolk, (6) the for- 

 mation of the indentation at the hinder margin of the blasto- 

 derm. In short, the addition of abundant yolk would seem in 

 the opinion of the writer to render the type of gastrulation of 

 Lepidosteus in every essential regard similar to that of Elasmo- 

 branch. On the other hand, the resemblances to amphibian 

 gastrula might be looked upon as only superficial ; its most 

 decided similarity seems but the result, of its approach to a 

 holoblastic condition, as seen for example in the formation of 

 a circular blastopore, and in the outward growth changes of 

 the early embryo (90-130 hours). To the Teleost gastrula its 

 resemblances may be summarized in (i) the origin of yolk 

 nuclei from the marginal blastodisc (= periblast), (2) the plan 

 of early segmentation, (3) the shield-shaped embryonic mass, 

 and (4) as will later be seen, the absence of a true neurenteric 

 canal. 



The establishment of the outward shape of the embryo has 

 its beginning in the stage figured from above and from the side 

 in PI. I, Figs. 17 and 18. The embryo is here a light-colored 

 cell-mass, whose rounded sides slope gradually to the yolk 

 below : a minute pit is the disappearing blastopore, marking 

 the tail region of the embryo ; the head end is sharply raised 

 and slightly projecting ; immediately in front of it a circular 

 and somewhat depressed area denotes the anlage of the vascu- 

 lar system. A sagittal section of an embryo of a slightly 

 earlier stage, PI. II, Fig. 32, is worthy of careful study : it 

 shows the condition of the closing blastopore, and the origin 

 of the middle germ layer. According to the writer's observa- 

 tions the closure of the blastopore takes place in the following 

 manner : at the stage figured in PI. I, Fig. 15, the rim of the 

 blastopore is becoming thickened and rounded, and the dorsal 

 lip is already considerably the thicker ; this mode of increase 

 in the thickness of the blastopore rim is continued until its 

 closure ; at about this time the relative thickness of both poles 

 may be well seen in the section, PI. I, Fig. 32 : the outer 



