1890.] The Mesoderm and the Calom. 885 
the embryo between the ectoderm and entoderm, thus becoming 
a continuous sheet or layer. This fact, that the mesoderm is a 
single anlage, needs to be specially emphasized. So far as 
known to me, there is not a single vertebrate which has been 
shown to lack this stage; but on the contrary, its occurrence is 
established for all classes, and by so many observers, that we may 
well assert that there are few facts in embryology better estab- 
lished. Later on the mesoderm becomes divided in the axial 
line, and a too exclusive consideration of this secondary condition 
has led to several theories of the mesoderm, which would hardly 
have been brought forward had their authors not neglected to 
take into account the earlier condition of the middle layer. Some 
of these theories are discussed below. 
After its delamination the mesoderm is a distinct layer, and 
grows independently, receiving no accretions from the other 
layers, except in the axial line, where it receives cells from the 
entoderm, and in the region of the primitive streak. The edge 
of the expanding sheet of mesoderm is free, as has been pointed 
out in the previous chapter, resting upon the yolk, but not fused 
with it. It is, therefore, it seems to me, impossible to admit that 
there is a peripheral ingrowth of tissues arising from the yolk, 
and entering the mesoderm to form the blood, etc. (compare 
below, Theories of the Mesoderm). 
The origin of the mesoderm in Amphioxus and invertebrates 
differs in many respects from that in vertebrates, and no attempt 
to establish the homologies of the processes throughout the ani- 
mal kingdom has been successful. I accordingly merely give a 
brief notice of the mesoderm of Amphioxus, adding a mention 
of the mesodermal bands of invertebrates. 
The ovum of Amphioxus is discharged from the body and 
impregnated externally; it is about 0.105 mm. in diameter, and 
as it contains only a small amount of yolk undergoes a holo- 
blastic segmentation, which results in a well-marked blastula 
stage (Fig. 24), followed by a gastrula stage. The gastrula elon- 
gates, the blastopore remaining open at the posterior extremity. 
Differentiations now take place, by which the ectoderm forms 
the axial anlage of the nervous system, and the entoderm pro- 
