4IO HOLMES. [Vol. XVI. 



of the anterior quadrants, which lie partly pushed into the cleav- 

 age cavity, finally lose connection with the ectodermic wall and 

 come to lie in the blastocoel. The cells from the two sides 

 nearly meet, forming a curved row of cells, the posterior ends 

 of which nearly meet the anterior ends of the mesoblastic bands, 

 which curve forward from the posterior side of the &gg. The 

 anterior row of cells forms what Wierzejski calls the secondary 

 mesoderm. As far as can be judged from the very brief descrip- 

 tion of this process in Wierzej ski's preliminary paper, the forma- 

 tion of the secondary mesoderm in Physa is very similar to, if 

 not identical with, its formation in Planorbis. The first cleav- 

 age of the cells of the third quartette in Physa is radial, as in 

 Planorbis, and the lower cell in the two anterior quadrants divides 

 horizontally into a right and left cell. The cleavage of the upper 

 cell is not described. The next division of the lower cells is 

 the same as in Planorbis, each giving off a small cell toward 

 the vegetal pole. The second pair of cells, 3<5''''', 3<5'^''', 3c^'*, 

 3r''"^', divide again in a radial direction, the upper cells going 

 to form mesoblast ; the fate of the lower cells was not deter- 

 mined. In Planorbis this cleavage is nearly transverse, the 

 outer cells being somewhat higher than the inner ones, and 

 probably corresponding to the upper cells (Mutterzellen) in 

 Physa. If the lower cells, 3<^="-'-' and 3-^""' •^ 3^'-'-'", ic'-'-'\ whose 

 fate Wierzejski did not determine, also form secondary meso- 

 blast, the cell origin of the secondary mesoblast in the two 

 forms would be identical. 



The last cleavages observed in the cells of the third quartette 

 were those of the large cells, 3^'' and 3^^ These divide bilat- 

 erally and in a nearly horizontal direction, a small cell being 

 given off from each at the outer end. Later these cells give 

 off another small cell in the same direction as before. After 

 these two divisions these cells are considerably reduced in size, 

 but at the time gastrulation begins they form a rather conspicu- 

 ous pair of cells, lying behind the nearly circular group of ento- 

 meres. The third quartette at this stage is composed of forty-two 

 cells. 



