2 2 DEAN. [Vol. XI. 



sides of the indentation, but by a mode of unequal growth 

 which caused the indentation to become less and less prominent 

 until it disappeared. In Fig. i6 the marginal indentation has 

 been retained and it is found to be lost only by the time the 

 diameter of the blastopore is reduced about one-half.^ In 

 Fig. 1 6 the margin of the blastopore is seen to be prominent, 

 and the outline of the embryo may be traced in the light- 

 colored region anteriorly. A sagittal section of this stage, 

 PL II, Fig. 31, shows the extent to which the lips of the blas- 

 topore are now separate from the underlying yolk ; it shows 

 clearly the point of union on either side of the yolk and the 

 cells of the inner germ layer (* of the figure). Its coelenteron 

 {c) is similar to that of amphibian, and the writer believes that 

 its mode of growth will upon careful study prove corresponding. 

 The section just noted shows in addition the boundary of the 

 segmentation cavity {s), and around the blastopore the partial 

 separation of the outermost epiblastic cell stratum, « Deck- 

 schicht.' Of this the differentiation is later found extended 

 over the entire surface of the egg. The yolk material slightly 

 protrudes through the blastopore as a rounded yolk plug. 

 No beginnings of the middle germ layer have as yet ap- 

 peared. 



From the conditions above described it seems to the writer 

 that the gastrulation of Lepidosteus might be looked upon as 

 an intermediate type, — primitive, moreover, inasmuch as from 

 its beginning till the time of the closure of the blastopore but 

 the two primary germ layers are concerned in its formation. 

 It is intermediate inasmuch as it presents a striking similarity 

 on the one hand to the gastrula of an Elasmobranch ; on the 

 other that it parallels the holoblastic type of amphibian, and 

 that it further indicates in its structures the most essential 

 characters of Teleost. It resembles the gastrula of Elasmo- 

 branch in (1) its meroblastic origin, (2) the presence of gene- 

 rally diffused yolk-nuclei in a granular superficial layer of 

 yolk, (3) the sharply marked character of the yolk surface 

 below the segmentation cavity, or more accurately beneath its 



^ Many variations occur; examples were noted in which the blastopore was 

 elliptical, ovate, or even slightly constricted in its ventral margin. 



