130 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 
entoderm” are said to be the primitive mesoblast cells, and the two 
posterior products ectodermal. ; 
It appears that the “central entoderm” cell of Grobben is probably 
the single entoblast cell to which Urbanowicz refers. The blastoderm 
cells lying laterally and anterior to the entoderm cell in Cyclops are 
said by Urbanowicz to give rise to mesenchyme, while Grobben in 
Cetochilus and Hacker in Cyclops find entoderm originating from cells 
in corresponding positions. It is probable that this contradiction arose 
from failure to follow the germ-layers into the ultimate organs. The 
figures of Cetochilus by Grobben and those of Cyclops by Hicker do 
not give conclusive proof regarding the fate of the cells which they con- 
sider endoderm. I have not seen the original figures by Urbanowicz. 
The differences between these authors will probably be adjusted when 
the later history of the mesoblast and entoblast is more accurately 
traced. 
The cell posterior to the “central endoderm ” cell in the thirty-two- 
cell stage of Cetochilus is said by Grobben to form the mesoblast and 
also to contain some ectoblast. This latter point must still be regarded 
as problematical, for Grobben’s figures do not give convincing proof. 
It is possible that the cell in question may be wholly mesoblastic, in- 
stead of only partly so. However, the important point is that this cell 
appears to originate in connection with the “central endoderm ” cell. 
Accordingly mesoblast in Cetochilus originates from entoblast ; a con- 
dition certainly existing in the case of the barnacle Lepas, and the 
studies of Urbanowicz make it appear probable that such is also the 
case in Cyclops. | 
Grobben’s (’79) account of the development of the phyllopod Moina 
agrees with Urbanowicz’s account of Cyclops and my own account of 
Lepas as to the formation of ectoblastic mesoblast from blastoderm cells 
bounding the blastopore laterally and anteriorly. But in a position 
corresponding to that of the entoblast cell of Lepas and Cyclops there 
is in Moina a “ primitive genital cell,” and the entoblast is said to be 
developed from a cell lying immediately posterior to it. It should be 
mentioned here that Samassa (’93), while agreeing essentially with 
Grobben’s description of cleavage stages, failed to find evidence of such 
early differentiation. With respect to this result it must be considered 
improbable that the visible peculiarities of the cells in the region of the 
blastopore in cleavage stages are without significance. It seems more 
probable that the peculiar features of certain cells do represent early 
differentiations, as Grobben claimed. The results of Samassa and 
