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E. A. SCHAFER. 
forward step in development. This is the separation or segre- 
gation of some cells from either one or both primary layers to form 
an intermediate set, which acquires greater proportional import- 
ance the further upwards we trace it in the scale of organisation. 
Since the cells of this intermediate set in most cases, and certainly 
in typical ova, are not of independent origin, but are derived in- 
directly from one or other of the two primary layers, it is clearly 
a mistake to regard the layer which they form as of equal im- 
portance with the other two — an idea which is distinctly implied 
by the name mesoderm’^ usually given to it. The two primary 
layers — the foundation membranes” of Huxley; ‘^ectoderm 
and endoderm” of Allmann — are distinct and well defined. The 
so-called mesoderm is often very different in these respects, so 
much so that in some animals parts which are looked upon as 
mesodermic by some morphologists are regarded as still belonging 
to the two primary layers by others. 
This mistake of regarding the mesoderm as of equal morpho- 
logical importance with ectoderm and entoderm has originated 
in the study of the development of the higher animals, in which, 
in many cases, the relative time of its origin has become shifted, 
so that it may begin to appear almost simultaneously with the 
primary layers. And even in some animals, not very high in the 
scale, the same shifting may be found. Thus, for example, in 
the Holothurian, the separation of some of the ectoderm cells to 
form part of the intermediate or mesodermic set has already 
begun, even before the commencement of invagination, and, there- 
fore, whilst the general development has not advanced beyond 
the blastophere stage. And the same shifting is carried to a 
still greater extent in Clepsine, where we find that even when the 
cleavage of the ovum has advanced but a step or two, certain 
portions are separated to produce, as the cleavage process con- 
tinues in them, the whole of the mesoderm. In this case, 
then, the third stage has begun almost as soon as the first stage 
itself. 
This premature separation or precocious segregation (Lan- 
kester) of parts which analogy would lead us to expect later is 
a very common feature in animal development. It is illustrated 
in Clepsine in a still more remarkable way, in the premature 
separation of the portions of the dividing ovum from which the 
cells of the nervous system are derived. And it is on account of 
this tendency which the phases of development exhibit to become 
shifted to periods earlier than usual for their appearance that I 
have compared them to the tubes of a telescope, for we can so 
much the better illustrate them in their relations to one another. 
Thus, in the case of the Holothurian, we should shift the third 
tube representing the formation of the mesoderm downwards, so 
