ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 285 



continuous membrane — the ectoderm •/ a true endodermic layer is 

 absent, though a superficial syncytium, the periblast, fuses with the 

 actual yolk surface. The mesodermic layer is represented by numerous 

 separate wandering mesenchymal cells, which may be seen, through the 

 transparent octoderm, moving over the surface of the periblast. 



The wandering cells begin to migrate away from the embryonic 

 shield when the embryo is about forty hours ; they come chiefly from 

 the caudal end ; they pass into the so-called sub-germinal cavity. 

 They are closely alike at first, but they soon exhibit differences. 

 Many become elongate spindle cells with delicate filamentous processes, 

 sometimes producing a stellate appearance. Others are more amoeboid 

 with mobile conical pseudopod-like processes. Somewhat later there 

 appear more slowly moving cells, circular in outline, with short thick 

 pseudopods. 



Those with the conical pseudopods accumulate black or brown 

 pigments. The black chromatophores gather along the walls of vessels, 

 and eventually form syncytia. The brown ones never become so 

 massive ; they also gather round the walls of the vessels, but they 

 never form a syncytium. The chromatophores never change into any 

 type of blood cell. , 



The elongate spindle cells become aggregated in linear groups 

 which form tubular vessels. The wall of the early blood vessels thus 

 formed is very irregular, with spaces between the component cells. 

 Corpuscles are often caught in these spaces or entangled in the filamen- 

 tous processes of the endothelial cells. It often looks as if endothelial 

 cells were changing into blood cells, but that is not the case. 

 Thoma's generalization that larger vessels arise from a network of 

 capillaries is not true in this case. Nor do vessels arise ontogenetically 

 as portions of the coelomic epithelium. The vascular lumen is 

 originally continuous with the segmentation cavity, never with the 

 coelom. 



The small globular cells with short pseudopods become erythroblasts, 

 and change about the fifth day from spherical to flattened and ellip- 

 soidal. Thus, in the same environment, superficially similar 

 mesenchyme cells differentiate in four directions of cytomorphosis — 

 two kinds of chromatophores, endothelial cells, and blood corpuscles. 



b. Histolog-y. 



Melanophores of Amblystoma Larvae.* — Henry Laurens has studied 

 ihe reactions of the melanophores in the larvae of Amhhj stoma punctatum 

 and A. opacum. Normal seeing larvae and eyeless larvae show different 

 states of the melanophores under identical conditions of illumination 

 and darkness, after they have been kept for some time under these 

 conditions. The melanophores of normal seeing larvge that have been 

 kept for longer than three to five days in bright diffuse daylight on an 

 indifferent background are contracted ; those of eyeless larvae are maxi- . 

 mally expanded. After prolonged darkness those of the seeing larvae 



* Journ. Exper. Zool., xviii. (1915) pp. 577-G38. 

 Jiine 21st, 1916 X 



