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THE AMEBIC AX NATURALIST [Vol. XLIX 



neither the position nor the direction of the cleavage 

 grooves enables one to predict the long axis of the em- 

 bryo, we are naturally led to look for other phenomena 

 which may be of significance. As stated in an earlier 

 paragraph my experiments showed that the head end of 

 the embryo is formed at, or very near, the active pole, 

 and since this area is the one in which cell division is 

 most rapid, it was concluded that the anterior end of the 

 embryo, which is the first to differentiate, was indicated 

 by this increased cellular activity. I accordingly stated 

 that an area of increased cellular activity indicates the 

 position of the head end of the embryo. As is well 

 known, this area can be located with the advent of the 

 first cleavage groove. 



While the head end of the embryo may thus be readily 

 located, the median plane of the body may lie in any one 

 of an indefinite number of meridians. The question 

 which now arises is which one of these meridians will 

 represent the median plane of the future embryo. 



The writer's studies on Rana, Bufo, Acris, Ambly- 

 stoma, Necturus have shown that in another portion of 

 the egg there is an area of smaller cells, and that this 

 area of smaller cells always marked the region of the 

 forthcoming blastopore. The blastopore in turn defi- 

 nitely fixes the posterior portion of the embryo. 



With the recognition of these areas of accelerated cel- 

 lular activity, the one at the active pole, indicating the 

 position of the future head of the embryo, the other at 

 the side of the egg, indicating the position of the forth- 

 coming blastopore, it necessarily follows that the median 

 plane of the embryo must coincide with a line passing 

 through the centers of the two. 



W T hen these observations were first published in 1898, 

 many questioned the existence of such a secondary area 

 of cellular activity. Yet a search through the literature 

 showed that such an area had been observed in many 

 groups of vertebrates. Lwoff found such an area at the 

 posterior end of the embryo of Amphioxus. The figures 

 of the segmenting blastodiscs of Elasmobranchs, given 



