46 PATTEN. [Vou. XII. 
But there are some very interesting phases of it seen in the 
embryos that are much reduced in size, and which may be 
best considered in this connection. The margin of the meso- 
dermic area is there frequently enlarged so that it becomes 
very conspicuous in surface views. It serves as a pretty 
safe index of the grade of development and degeneration, 
and also to identify the posterior end of the body in embryos 
reduced to such low terms as those in Pls. VI and VII. This 
structure is formed by the fusion of the peripheral margin 
of the mesoblastic somites with the ectoderm and with the yolk 
cells! A long primitive, streak-like thickening ts thus formed, 
by the proliferation of which the germ layers are extended laterally 
in exactly the same way that we are familiar with as occurring 
at the posterior end of the body. 
There is a great difference in the lateral extension of the 
segmented portion of the mesoderm. In normal embryos it 
may reach the very margin of the mesodermic area, while in 
abnormal cases it may not extend beyond the appendages. 
But there is universal agreement in normal and abnormal 
embryos in the union at the periphery of ectoderm, mesoderm, 
and yolk cells. 
The lateral growth then of a metamere is comparable with 
the posterior apical growth by which the body of the embryo ts 
encreased in length. 
In normal embryos, the lateral margins of the mesoblastic 
segments, when first formed, are quite indistinct, Fig. 1, but 
they soon fuse to form the conspicuous marginal thickening just 
described, Figs. 2-4, m.a. The egg being nearly spherical, the 
anterior and posterior portions of the margin have a shorter 
distance to go in order to unite with each other in the median 
line than the middle portions. For this reason and others that 
we have already discussed, the margins, following the path of 
least resistance, tend to form anterior and posterior loops, which 
finally meet in front of and behind the embryo, PI. I, Fig. 5, 
forming a rounded, mesodermic area in the centre of which the 
1 We cannot for lack of space discuss the origin and history of the mesoderm 
here in the detailed manner that it deserves. We can only point out briefly the 
more important facts that bear on the subject matter of this paper. 
