290 HELEN DEAN KING. 



posteriorly, the archenteron is surrounded on its ventral, lateral 

 and lateral-dorsal surfaces by large, rounded, faintly staining 

 yolk cells which contain very little, if any, pigment ; the mid- 

 dorsal wall, on the contrary, is formed of a single layer of much 

 smaller, rectangular cells which are very heavily pigmented on 

 the side bordering the archenteron. This layer of cells, which 

 I shall call " the dorsal plate," is broadest in the posterior part of 

 the embryo, where, in transverse sections, it appears as a nearly 

 straight line of cells covering about two-thirds of the mid-clorsal 

 surface of the archenteron. More anteriorly the dorsal plate 

 gradually becomes narrower, until it finally disappears completely 

 in the middle of the embryo. The archenteron in front of this 

 region is entirely surrounded by large yolk cells. 



The outer cells of the dorsal plate, instead of grading into the 

 yolk cells as one might expect, are found to be directly continu- 

 ous with the lower layer of mesoderm. There is, therefore, in 

 this region an abrupt change from the small, deeply pigmented 

 cells of the dorsal plate to the large yolk cells which form the 

 lateral and ventral walls of the archenteron. At no stage in the 

 development of the embryo have I ever found any transitional 

 stages between these two different kinds of cells. The cells of 

 the dorsal plate resemble, in all respects, the cells forming the 

 outer surface of the embryo, being of the same size and shape 

 and containing about the same amount of pigment. From the 

 results which I obtained in my study of the gastrulation of the egg 

 of this species (King, 12), it seems highly probable that the cells 

 composing the dorsal plate were invaginated from the surface of 

 the egg during the formation of the blastopore, and, consequently, 

 they have had a very different origin from the cells forming the 

 lateral and ventral walls of the archenteron which are all derived 

 from the yolk portion of the egg. 



When the medullary folds are closing, the mesoderm in the 

 posterior region is still connected, for a short distance, with the 

 cells forming the dorsal wall of the archenteron, and the noto- 

 choro has not yet extended into this portion of the embryo. Fig. 

 3 shows a portion of the section through the region where the 

 notochord has just been cut off from the mesoderm. This sec- 

 tion corresponds in its position in the embryo with the position 



