108 GEOEGE W. FIELD. 



of the mesenchyme could be referred, but with negative 

 results. 



Mesenchyme Formation. — About twelve hours after fer- 

 tilisation cleavage is completed, and results in a ciliated 

 cceloblastula, which spins around within the egg membrane ; 

 soon by the rupture of the egg membrane the blastula 

 becomes free-swimming, and immediately seeks the surface of 

 the water. Then appear the first traces of mesenchyme for- 

 mation. In the region of the more columnar cells, the future 

 entoderm, one and then more ceils push out into the seg- 

 mentation cavity, and become amoeboid mesenchyme-cells. 

 Usually the entire cell pushes out from the entoderm, but 

 frequently there is a transverse division, and only the inner 

 half becomes amoeboid (fig. 6). Somewhat later this portion 

 of the sphere becomes flattened (fig. 7) and is gradually 

 invaginated. During the progress of the invagination amoe- 

 boid mesenchyme-cells in great numbers wander into the 

 segmentation cavity from the walls of the invaginated portion 

 (fig. 8) ; some of these amoeboid cells are formed by division 

 of the entoderm-cells as above described, while the majority 

 are in no way distinguishable from the cells which remain as 

 the permanent entoderm. The stage where only one, two, 

 or three mesenchyme-cells are present is quickly followed by 

 the appearance of others which arise from any part whatever 

 of the entodermal area (figs. 6, 7, and 8), and I am led to 

 believe that the condition in Asterias vulgaris is the same 

 as that found by Metschnikoff", and by Korschelt (21) in 

 other Echinoderms, i. e. the absence of two bilaterally sym- 

 metrical " Urmesenchymezellen " — a view in opposition to that 

 of Hatschek, Selenka, and Fleischmann. 



My observations on Asterias vulgaris in regard to the 

 time of the beginning of mesenchyme formation relatively to 

 the process of gastrulation difl"er from those of Metschnikoff 

 in Astropecten, inasmuch as I find that the mesenchyme 

 formation precedes and continues throughout the progress 

 of the invagination. No traces were found of the bilaterally 

 symmetrical rows of cells comparable to the mesoblastic 



