202 HUBERT DANA GOODALE 



which lie above the dorsal lip at its first appearance. Whether 

 they be called ectoderm, animal cells or what not, is of no import- 

 ance for the present, though most authors would doubtless call 

 them ectoderm. As to the source of the rest of the archenteric 

 rQof, there is relatively little agreement, as shown by the papers 

 of Strieker '62, Goette 75, Moquin-Tandon 76, Hertwig '83 

 and '03, Schultze '88, Schwink '89, Houssay '90, Robinson and 

 Assheton '91, Jordan '93, Lwoff '94, Morgan and Tsuda '94, 

 Eycleshymer '95, Samassa '95, Brauer '97, Gronroos '97, Wilson 

 '00, Adler '01, King '01, Ikeda '02, Morgan '02, Brachet '03 and 

 Hilton '09. 



Mesoderm 



The mode of origin of the gastral mesoderm in Spelerpes is 

 perfectly clear, although it apparently appears in some eggs at 

 an earlier stage of development than in others. The youngest 

 embryo I have, in which there is any trace of mesoderm is one in 

 which the archenteric invagination has reached the stage shown 

 in fig. 33. A cross-section of such an egg through the blasto- 

 pore region shows that the peristomal mesoderm, composed of 

 small polygonal cells, forms the roof, sides and floor of the archen- 

 teron. Anteriorly, the archenteron roof consists of several layers 

 of small cells, which extend slightly below the level of the floor 

 of the archenteron. This extension of small cells I take to be 

 precociously differentiated mesoderm. However, there is only 

 one egg in which I have seen this phenomenon so clearly at this 

 period. 



Cross sections of slightly older stages usually fail to show a 

 similar extension of small cells, although the cells of this region 

 may not be quite as large as the neighboring yolk cells, which 

 are usually separated from one another (fig. 36) . The separation 

 of cells is merely an expression of their migration. At this time, 

 the cells in the dorsal region have again arranged themselves into 

 a definite layer of irregular yolk cells, but at the sides, where 

 invagination started later, this re-arrangement is not yet com- 

 plete. 



