GASTIiUr.ATION IN BIHDS. 
149 
obstacle, and is not in any way compai-able to an actual 
flowing of a fluid past a fixed and solid objectd 
If the segmented egg of a frog, or the segmented blasto- 
discof a bird, were perfect fluids, tlien the objection would be 
a fatal one. Or if the segmented egg Avere like a heap of 
shot, then also such a mass could flow slowly past a fixed 
object Avithout producing any visible rippling. But the seg- 
mented ovum is not a pile of separated cells. The cells, or 
many of them, are in continuity by means of viscous cyto- 
plasmic strands, and the Avhole mass is by no means a perfect 
fluid. 
If a bristle is inserted into the yolk-plug of aii egg of 
Rana temporaria during the crescent stage of the blasto- 
pore close up against the advancing dorsal lip of the blasto- 
])ore, there is no tendency either for the advancing lip to be 
divided by the bristle, nor for the bristle to be driven through 
the yolk plug-cells, thus proving the absence of anything 
approaching a perfect fluidity of either the ectodermal or the 
endodermal layer of cells. 
I may remark here that when one of iny experiments suits 
Patterson’s purpose he accepts it ! (p. 115). If cells can sweep 
past to concresce when the bristle is placed to one side, the 
fact that my bristle, when placed in the area opaca in the 
posterior margin, did not appear in the embryo, is no proof 
that the cells that do form the embryo have not come sweep- 
ing past the bristle. 
Personally I cannot agree Avith Patterson’s vicAv (p. 109) 
that concrescence and gastrulation are tlitferent phases of the 
same process. Gastrulation is the formation of the gut 
cavity. If this formation is accompanied by the production 
of a blastopore (which is by no means always the case, e.g. 
Hydrozoa, probably all mammals — I Avould e\'en add all 
' These remarks refer to Rana temporaria only. There is much 
variation in the viscosity of amplii))ian eggs. I haA’e failed Avith the 
B'gmented egg of Triton cristata. The eggs of Bufo are also less 
snitahle than those of R. temporaria owing to greater fluidity. The 
results obtained from the chick and Rana temporaria I Ijelieve to he 
ipiite reliable. 
