328 JORDA N. [Vol. VIII. 



stage. The section through the egg of Tj'iton tcuniaius, de- 

 picted by Hertwig (Fig. i, PI. XIII) does not at all correspond 

 with what I find in median sections through the eggs of Diemyc- 

 tylus at this period. 



External appearances . — Beyond a slight increase in the 

 diameter of the egg, due, doubtless, to the formation of the 

 segmentation cavity, there is for some time no visible external 

 change except decrease in size of the cells consequent upon 

 their multiplication. The first signs of gastrulation appear 

 about fifty-four hours after the Qgg is laid, the temperature of 

 the water being about 18° C. There is first a slight depression 

 of some of the cells in the middle of the lower pole ; this be- 

 comes more and more pronounced, and finally results in an 

 irregular pit, such as shown in Fig. 34. This pit grows deeper 

 and its edges become more sharply outlined till there is a deep, 

 nearly oblong depression extending in the direction A-B, 

 Fig. 34. This depression is now at right angles to the future 

 axis of the embryo. Almost as soon, however, as the depres- 

 sion assumes this form, faint sunken lines may be seen running 

 off from the ends of the depression at an obtuse angle. The 

 point of junction of these lines of sinking with the original 

 area of depression is at first perfectly clear, and the angle 

 is well marked, but very soon the angle is obliterated and 

 the depression takes on the rounded semi-lunar appearance 

 which has become familiar by repeated portrayal (Fig. 35). 

 The horns of this half-moon gradually approximate, and in 

 about twenty-four hours after the appearance of the depression 

 the circle of the blastopore is complete (Fig. 36). 



The more intimate phenomena of the formation of the blast- 

 opore must now be considered. 



Invagination. — The view that the archenteron of the amphibia 

 is formed by the infolding or ingrowth of cells that originally 

 were external has been the one very generally upheld by the 

 students of amphibian development. This conventional opinion 

 has, however, been opposed by Moquin-Tandon ('76) and by 

 Houssay ('9o), and has recently been attacked with spirit by 

 Robinson and Assheton ('9i). These dissenters from the in- 

 vagination hypothesis believe in its stead, "that the archen- 



