226 EICHARD ASSHETON. 



formation of the primitive archenteron is by a process of split- 

 tings and is the direct effect of the primary centre of growth ; 

 whilst the continuation of the cavity produced by an overgrowth 

 is the direct effect of the secondary centre of growth^ producing 

 the elongation of the animal. 



The splitting process in the frog corresponds in results to the 

 invagination process of Amphioxus, while the overgrowth of 

 certain parts of the white pole of the ovum of the frog by the 

 dorsal, and subsequently lateral and ventral lips of the blasto- 

 pore, together with the continuation of this process in the for- 

 mation of the tail, corresponds to the elongation of the gastrula 

 in Amphioxus, by means of what Hatschek called the polar cells. 



I shall now attempt to explain what I believe to be the actual 

 method in which the splitting is brought about. 



The frog's egg segments, as has been described by many 

 observers, more rapidly at one pole than the other. This 

 is, I think, universally supposed to be due to the greater accu- 

 mulation of yolk granules at the " lower '^ pole, which thereby 

 hinder the segmentation activity at that pole. 



If we admit that '' yolk " determines the inequality of the 

 process known as segmentation, we must admit it also in the 

 case of each cell. If it is true of the segmented ovum, it is 

 equally true of the unsegmented ovum. To say that yolk 

 being more plentiful in one part of a cell than in another 

 hinders the activity of the protoplasm, is the same as saying 

 that a cell divides into two parts, which in magnitude are in 

 inverse ratio to the purity of the protoplasm contained. In 

 other words, the result of a simple process of cell division, such 

 as we see in the segmenting ovum, is two cells equally balanced 

 as regards protoplasmic energy. 



Pig. 15 on PL 24 is a diagram of a vertical section of the 

 unsegmented ovum of the frog. 



The circles 1 to 7 represent diagrammatically what I imagine 

 to be the distribution of yolk, as determined from a considera- 

 tion of the segmented ovum. 



The space No. 1 is that region in which segmentation is 

 most retarded, and so presumably the region in which yolk is 



