188 ARTHUR K. SHIPLEY. 



become columnar. In this anterior region the mesoblast soon 

 unites ventrally. In the posterior region the ventral union of 

 the mesoblast is delayed, the lateral plates of mesoblast lying 

 between the yolk-cells and the epiblast end in a free edge, and 

 until these edges unite, the yolk-cells are in contact with the 

 epidermis ventrally. 



In the region between the liver and the last gill-slit the 

 mesoblast splits at about the fifteenth day into a somatic and a 

 splanchnic layer; between the two a well- developed body- 

 cavity appears. The former layer lines the body wall, the 

 latter envelopes the alimentary canal. It forms a dorsal mesen- 

 tery supporting that structure, and a well-marked ventral 

 mesentery of considerable depth connecting the ventral wall of 

 the intestine with the body wall. It is in this ventral mesen- 

 tery that the heart is developed. The two layers forming the 

 mesentery fuse dorsally and ventrally, but separate from one 

 another in their middle, forming a cavity which is the lu- 

 men of the heart (fig. 24). Subsequently both the mesentery 

 connecting the heart with the alimentary canal — the meso- 

 cardium — and the ventral one connecting the heart with the 

 ventral body wall, atrophy and the heart lies as a tube uncon- 

 nected with the surrounding structures (fig. 25). 



From the fact mentioned above that the mesoblast behind 

 the heart has not split into somatic and splanchnic layers nor 

 united ventrally, it will be seen that the cavity of the heart 

 communicates posteriorly with the space between the ventral 

 yolk-cells and the epidermis. Such a space would be equiva- 

 lent to part of the segmentation cavity. Soon after the heart 

 is formed such a space arises, and at once becomes crowded 

 with cells destined to form blood-corpuscles (fig. 26) . At first 

 I was inclined to think that these cells were budded off from 

 the volk-cells, but more careful observation has led me to 

 believe that they originate from the free edge of the lateral 

 plates of the mesoblast, which as I mentioned above are 

 growing down between the yolk-cells and the epiblast. These 

 corpuscles are oval with large nuclei, and they usually contain 

 at first one or two yolk granules which they soon absorb. 



