THE DEVELOPAIENT OF LEPlt)OSlUEN PARADOXA. 15 



blastopore is growing bodily downwards, wedging itself in 

 between the capsule and the large yolk-cells, and causing 

 as it does so the latter to iuvaginate into the floor of the 

 archenteron. 



(/3) The frequency of mitotic figures in the region over- 

 lying the archenteron, and more especially in the dorsal lip, 

 appear to indicate active growth of this region, and conse- 

 quent backward migration of the blastoporic lip. 



(y) During the later stages of gastrulation I find that the 

 angle between the lines A and B (passing from the centre 

 of the section to the tip of the archenteron and to the margin 

 of the small-celled area respectively) remains nearly con- 

 stant, and the increase in the angle A O C corresponds fairly 

 closely with the diminution in the angle COB. 



This seems to suggest that the line C is gradually swing- 

 ing through the arc between A and B. Otherwise we must 

 believe that the lines B and A are swinging with equal 

 velocity in a clockwise direction. It appears to me from 

 study of my sections that this is not the case, the forward 

 movement of the point B being very slow compared with the 

 advance of the archenteric tip. 



(S) The cells of the ventral wall of the archenteron are 

 continuous, without an;f visible change in character, with the 

 large yolk-cells lying exposed on the outer surface of the egg 

 below the blastopore. 



On the whole, then, I believe that the evidence, such as 

 it is, points to the view that the main factor of the increase 

 in length of the archenteron during ihis last stage is the 

 downgrowth of the blastoporic lip. 



While these processes of formation of the archenteron 

 have been going on the area of yolk-cells exposed has been 

 gradually reduced, dorsally by the growth of the blasto- 

 poric lip, elsewhere by the gradual encroachment of the 

 small-celled area. This spreading of the small cell margin 

 over the yolk-cells is most rapid in the neighbourhood of the 

 blastoporic lip, least so at the point opposite to this. In this 

 latter region the superficial layer of small cells passes into a 



