340 JORDAN. [Vol. VIII. 



(p. 400). In Luvibiicns the blastopore closes from behind 

 forwards, in RJiyncJiehnis and Clcpsmc from before backwards. 



To what, then, shall we attribute the variations in the closure 

 of the blastopore .-* Evidently they can be due to no phylo- 

 genetic necessity, but rather to slight, unimportant individual 

 differences in the rapidity of cell proliferation and movement 

 at different parts of the blastopore rim, and may perhaps in the 

 final analysis rest upon trivial mechanical causes. 



One important fact, however, is brought out into sharp relief 

 by the work of all investigators of the amphibian blastopore, 

 namely, that the blastopore is closed at least in part by closing 

 in of the lateral lips along a median line. I cannot regard it 

 as an objection against the theory of concrescence of the verte- 

 brate embryo that the blastopore does not always close by 

 retrogressive fusion of its lips. On the contrary, I think 

 variations such as we have noticed are distinctly the outcome 

 of secondary and fluctuating conditions. Whether the blastopore 

 closes "from behind forwards " or "from before backwards," 

 or in both directions, does not appear to have great significance, 

 and does not in the least affect the salient fact of coalescence 

 of the lateral lips along a mid-line. 



Neuropore and Anns. — It is evident, from what I have 

 already stated, that the relative position of neuropore and 

 anus cannot be constant. I am thoroughly convinced, after 

 a careful examination of many preserved eggs, and after obser- 

 vation of the blastopore closure in the living ^gg, that the 

 anus may lie in almost any portion of the original mid-line 

 through the blastopore. Fig. 42 shows the greatest sepa- 

 ration of neuropore and anus that I have ever observed. The 

 two openings usually lie much closer together, at a distance 

 represented by about one-third of the interval shown in Fig. 

 42. The examination of preserved specimens inclined me at 

 first to believe that the neuropore was pushed back from some 

 such extreme initial position as that represented in the figure, 

 but study of the living ^gg showed me that such is prob- 

 ably not the case, and that the gradations I observed 

 were individual variations and not consecutive stages. The 

 neuropore and anus, then, have their position, relative and 



