FORMATION OF GERM-LAYERS IN CRANGON VULGARIS. 359 
structure ; and at the same time the endodermal cells arrange 
themselves in the form of a tube, closed posteriorly and 
dorsally, where it is in contact with the former blastoporic 
area, and widely open anteriorly, where its funnel-shaped 
mouth embraces the yolk, its ventral wall being continuous 
with the ventral sheet of cephalic endoderm already described. 
The ventral plates commence at this time to increase in size ; 
and at the posterior extremity of each a pair of large cells is 
found (fig. 30, m. 6. and e. b.), one cell being especially related 
to each of the two layers which form the plate. These cells 
are evidently homologous with the “ Knospungszellen ” of 
Reichenbach, and may be spoken of as ectoblasts and meso- 
blasts respectively. — 
I regret that in spite of numerous attempts I have been 
unable to find an embryo which showed the exact mode of 
origin of these large cells; but there can, I think, be little 
doubt that they arise as specialisations of cells which already 
existed in the ventral plates during previous stages, and not 
by the addition to these plates of new cells derived from the 
endoderm. 
When the embryo has reached the stage represented in fig. 
19, the budding cells have increased in number, and each has 
a fairly definite relation to a band of cells on the ventral side 
of the embryo. The flexure of the thoracico-abdominal 
papilla prevents these cells from appearing in surface views; 
but their relations will be apparent from the horizontal longi- 
tudinal sections of the papilla which are represented in figs. 
31—33, and which cut the embryo in planes parallel to the 
line z y in fig. 84, and perpendicular to the plane of the paper. 
Of these sections, fig. 31 represents the most dorsal, fig. 33 the 
most ventral. In fig. 31 the tubular portion of the posterior 
endoderm is well seen; and its perfect continuity with the cells 
of the first-formed endoderm, which by this time form a 
fairly regular layer on the ventral surface of the yolk, is also 
evident. 
The section passes through a single mesoblast on each side, 
which les at the extremity of a considerable plate of well- 
