DEVELOPiMENT OF PHRYGANIDS. 575 



plate have undergone division, and one of the products of that 

 division having assumed amoeboid characters, is now in the 

 process of freeing itself from the blastoderm preparatory to 

 migrating into the yolk. There can be no doubt as to which of 

 these explanations is correct. There is not the slightest 

 evidence of the free formation of cells within the yolk, and 

 their intimate connection with the serosa and ventral plate 

 points clearly to the cells of these structures as the source of 

 their origin. These so-called yolk or endoderm cells, which 

 later become somewhat modified in size and appearance, are 

 gradually distributed through all parts of the yolk and ulti- 

 mately form the epithelial lining to the mesenteron. 



Stage 4. — This period extends from the completion of the 

 embryonic membranes up to the appearance of the appendages ; 

 it is marked by the segmentation of the mesoderm. 



The germinal band is now a thick layer of cells, extending 

 over about three fourths of the circumference of the eg^, and 

 continuous at its periphery with the amnion, which meantime 

 has become very thin. Surrounding both amnion and germinal 

 band, may be seen the serosa, with its large granular nuclei, 

 from which the amnion can readily be distinguished by the 

 small size of its nuclei. The two cell layers ultimately become 

 very closely united, although never completely fused, since in 

 sections of the embryo just previous to revolution it can 

 readily be seen in many places, especially near the nuclei of 

 the amnion, that tliey are separate membranes. 



The amnion always maintains its continuity with the extre- 

 mities of the body, so that the amniotic cavity remains com- 

 pletely closed, even up to the time of the rupture of both 

 amnion and serosa. The extreme tenuity of the amnion at this 

 stage is in marked contrast with its appearance when first 

 formed, it being then as thick as the ventral plate. 



The invagination which gave rise to the mesoderm and 

 part of the entoderm occurred during the formation of the 

 embryonic membranes, and disappeared when the latter had 

 completely fused with each other, hence no trace of an invagi- 

 nation is to be observed during this stage. It is of great 



