132 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 
such conditions may be cited Daphnia, according to Lebedinsky (91) ; 
Moina and Daphnia, according to Samassa (’93); and many higher 
Crustacea. 
Such an origin of mesoblast and entoblast is not necessarily opposed 
to the account which I have given of the germ-layer formation of Lepas, 
for differentiation, though not observable, may yet occur in the cases 
mentioned. Were there not in Lepas peculiarities by which the cells 
can be distinguished at an early stage, the immigrating mass of cells, 
composed of entoblast, and of primary and secondary mesoblast, would 
be correctly described as mes-entoblast, out of which the two layers 
become later visibly differentiated. If the entoblast cells of Lepas were 
completely separated from the yolk-mass, as is the case in many other 
Crustacea, it would perhaps be impossible, in the absence of the easily 
recognized yolk-laden entoblast, to trace the lineage of the mesoblast 
independently of the entoblast, and in such conditions it would be nec- 
essary to consider the immigrating mass of cells as mes-entoblastic. It 
is probable that some such conditions obtain in some of the Crustacea 
in which a mes-entoblastic immigration is said to occur. At any rate, 
germ-layer formation in such cases agrees in essentials with that 
observed in Lepas. Grobben’s (’79) study of Moina suggests that in 
this genus, at least, the immigrating mass of mes-entoblast may be not 
entirely undifferentiated as Samassa (’93) supposed. 
There is some evidence that the comparison between Lepas and cer- 
tain higher Crustacea may be carried still farther than the suggestions 
offered in the preceding paragraph. In Astacus, according to Reichen- 
bach ('86), the mesoblast originates at the anterior margin of the 
blastopore, where the ectoblast joins the entoblast. Reichenbach dis- 
tinguished in the invagination both yolk-absorbing cells (vitellophags), 
which enter into the yolk-pyramids, and also the cells forming the 
entoderm plate. All these cells are said to enter into the mesenteron 
and liver lobes, and hence the invagination is entoblastic. However, 
MecMurrich (’95, pp. 135, 136) reviews the evidence and suggests that 
the yolk-pyramids give rise to some mesoblast. If this proves true, the 
invagination is to be regarded as mes-entoblastic ; but, in addition to 
mesoblast so formed from entoblast, other mesoblast cells certainly 
originate from the blastoderm in front of the invagination. It follows 
that there are, as regards origin, two kinds of mesoblast — ectoblastic 
and entoblastic. 
In other accounts of development of the higher Crustacea there are 
suggestions of such a double origin of mesoblast, but there is as yet 
