248 BULLETIN OF THE 



ferences, however, are far greater than those which exist between Rana 

 and Bufo, and wdl require for their presentation a fuller treatment than 

 was given in the case of the latter genus ; but the development in all 

 three genera is sufficiently similar to allow the recognition of the same 

 successive stages, based upon the degree of complication exhibited by 

 the pronephros. 



Stage I. 



Plate VI. Fig. 48. 



In embryos of this stage, the two lateral medullary folds have just 

 fused to form the neural tube. The embryos have a slightly elongated 

 form and measure about 3.7 mm. in length. They are slightly more 

 advanced than the embryo of Amblystoma represented by Bambeke 

 ('80, Blanche XI. Fig. 35). The eggs from which I derived my series 

 of embryos had been deposited for a variable length of time before they 

 were collected, and I am unable to give the ages of the several stages. 1 



The general arrangement of the germ layers (Blate VI. Fig. 48) is 

 similar to that which I have described for Bana and Bufo. The ecto- 

 derm (ee'drm.) consists in general of a single layer of cells, each of which 

 has the form of a cube slightly flattened. Scattered ectodermal cells 

 form an incomplete deep layer, which may gain in some regions, e. g. in 

 the head, a very considerable development. The outer face of each 

 ectodermal cell possesses a thin layer of pigment, but this is by no means 

 so dense as in Bana and Bufo. At this stage yolk spherules are abun- 

 dant in all the cells of the ectoderm. 



The entoderm has nearly the same arrangement as in Bana, but the 

 yolk cells are relatively more abundant, and the lumen of the gut is 

 narrower. In the anterior region, the chorda consists of a simple fold in 

 the dorsal roof of the intestine ; but in the posterior portion of the body 

 it is represented by a single row of high columnar cells, which form a 

 layer convex from side to side towards the lumen of the intestine. This 

 layer is the one which 0. Hertwig ('83) has named the chorda-ento- 

 blast. The cells of the yolk entoderm are in general the largest in the 



1 A quantity of the eggs of Amblystoma punctatum Linn, raised in the laboratory 

 during the present season reached the several stages as follows : Stage I., 5 days; 

 Stage II., 5 days, 12 hours; Stage III., 6 days, 15 hours; Stage IV., 7 days, 15 

 hours ; Stage V., 8-14 days ; Stage VI., 15-20 days. These figures are only approx- 

 imate, and between Stages II. and V. the individual variation is frequently more 

 than sufficient to cover the entire interval between two successive stages. The 

 temperature of the water varied somewhat during the period, but I believe that 

 10 or 11° C. would be a fair average. 



