GROWTH IN LENGTH OF THE FEOG EMBRYO. 227 



most abundant. The space No. 2 contains less yolk to a given 

 area than No. 1, No. 3 less than No. 2, and so on. 



For the sake of simplicity we may regard the outer space 

 only. This may be supposed to contain protoplasm of a 

 uniform degree of purity. Accordingly division of this space 

 will be such as to produce two spaces whose areas are equal. 

 This is about the spot marked by the line {a), and \vill repre- 

 sent the third furrow of segmentation, that is the first hori- 

 zontal furrow. 



Similarly, the next horizontal furrows will be about the 

 spots b b, the next at c c c c, the next atdddddddd, and 

 so on ; always resulting in a balance of protoplasmic energy 

 on each side of the furrow. 



In this way the frog's egg becomes segmented more and 

 more rapidly in the upper hemisphere than in the lower. For 

 a considerable time there is an almost complete absence of 

 horizontal furrows in the lower hemispheres. 



This point is very well seen in Ume Tsuda^s figures iv, v, of 

 Plate 24, ' Quart. Journ. Micr. Sci./ vol. xxxv, part 3. 



Another effect is that as segmentation proceeds there is a 

 continual increasing disparity in size between the cells of the 

 black pole and those of the white. Whereas at first the 

 superficial area of the cells of the extreme upper pole bears to 

 the superficial area of the cells of the extreme lower pole the 

 ratio of 1 to 2, at the time of the commencement of the blasto- 

 pore it bears the ratio of 1 to 5, 



In this Way there is a gradual apparent creeping of small 

 (black) cells over the surface of the egg — though in reality it 

 is conversion of large cells into smaller in situ, as, I believe, 

 is now generally accepted. 



My diagram fig. 15 gives the idea of no segments in the 

 Avhite or lower hemisphere of the ovum. This is because it 

 deals only with horizontal furrows. 



The segmentation energy may be said to produce its effects 

 along the area of least resistance. Is it not possible that the 

 commencement of the archenteron may be a continuation of 

 this same process ? 



