THE FOEMATION OF THE LAYERS IN' AMPHIOXUS. 327 
primitive streak.^ The division of the solid in-growth into 
central notochord and lateral mesoderm masses corresponds 
to the system of grooves found in the roof of the archentei'on 
of lower Amniota. 
When we pass to Mammalia, we find that the exact 
sequence of events in the earliest stages of segmentation is 
most imperfectly known. The egg is described as forming a 
solid mass of cells or morula, in one side of which vacuoles 
appear, which lead eventually to the formation of a space 
filled with fluid. When the egg reaches the uterus it is in 
the form of a vesicle with a wall consisting of a single layer 
(Rauber’s layer), attached to one side of which is a mass of 
cells, called by Hubrecht (18) the embryonic button. 
From the superficial layer of the embryonic button the 
ectoderm of the embryo is differentiated, and Rauber’s layer 
over the region of the button disappears. The development 
of the embryo, once this has taken place, closely resembles 
that of the chick, so that the principal new question presented 
for our consideration is the meaning of Rauber’s layer. I 
suggest that it is nothing else than the sheet of ectoderm 
which in the eggs of Amphibia sweeps round the ventral 
surface of the egg, and eventually forms the ventral lip of the 
blastopore. Owing to the loss of yolk by the mammalian egg 
it is absolutely necessary for its survival that at the earliest 
possible moment it should be provided with a covering of 
cells capable of assimilating nourisliment from the womb-wall. 
Hence the normal spreading of ectoderm, i. e. growth of the 
ventral and posterior lip of the blastopore, takes place long 
before there is a trace of the dorsal lip, and hence there 
is differentiated a special layer of ectoderm cells for this 
purpose even over the embryonic area. That this is a 
secondary phenomenon is shown by Hill’s observation that in 
^Marsupials this is not the case, but that here the embryonic 
’ Wilson and Hill (38) describe in Ornithorliynclivis a very 
anomalous condition of affairs, viz. the co-existence of an open blasto- 
pore and of a i)rimitive streak some distance behind it. 
VOI-. 54, PART 3. — NEW SERIES. 
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