74 



Mr. F. G. Heathcote. 



[Jan. 21, 



third day the first formation of the blastoderm begins. Early on the 

 third day some of the segmentation masses make their appearance on 

 the outside of the ovum at different parts, and there undergo rapid 

 division, the resulting cells spreading out to form the blastoderm. 

 At the close of the blastoderm formation, the ovum consists of an 

 external layer of flat cells — the epiblaast — with deeply stained oval 

 nuclei, these cells being continuous on the one hand with one another, 

 and on the other with the cells in the yolk by means of fine processes 

 of protoplasm. The cells in the interior of the yolk are the direct 

 descendants of the first segmentation masses. They constitute the 

 hypoblast. 



The fate of these Hypoblast cells is various ; some of them are 

 employed in the formation of the mesoblastic keel which I am about 

 to describe, that is, in the formation of the splanchnic and somatic 

 mesoblast. Another part gives rise to the hypoblastic lining of the 

 mesenteron, while a third part remains in the yolk after the 

 mesenteron is formed, and gives rise to mesoblast cells which are 

 employed in the formation of various muscles and of the circulatory 

 system. 



With regard to the retention of the primitive connexion of the 

 cells of the ovum until this stage, nothing of the sort has, I believe, 

 been described before except by Sedgwick in Peripatus. The most 

 important part it seems to me is not the connexion of cell to cell but 

 of layer to layer by means of processes of the cells. 



About the middle of the fourth day several of the hypoblast cells 

 approach the epiblast in the middle line of what will eventually be 

 the ventral surface of the embryo. This is the first beginning of a 

 mesoblastic keel such as Balfour has described for Agelena 

 labyrinthica. When a fair number of these cells are assembled in the 

 middle line of the ventral surface a change takes place in the cells of 

 the epiblast just outside them. They become more rounded, their 

 nuclei become round ; in fact they come to resemble the cells which I 

 have described as assembling immediately below them. 



The epiblast cells in the middle ventral line after altering their 

 shape increase by division and take a considerable share in the formation 

 of the keel. The hypoblast cells below them also increase, and on the 

 fifth day the mesoblastic keel is complete. Both epiblast and 

 hypoblast have taken part in the formation of this keel. 



At the end of the sixth day the keel is still present, but the cells of 

 which it is composed are becoming elongated in the direction parallel 

 to the surface. At the same time they continue to multiply and 

 spread themselves out so as to form two definite layers within the 

 epiblast. These are the splanchnic and somatic layers of the 

 mesoblast. The cells of the splanchnic and somatic mesoblast are 

 connected. 



