48 PATTEN. (Vou. XII. 
is usually distinctly indented in such cases, as though the 
concrescence were proceeding still farther backwards. 
Various modifications of the different phases of concrescence 
are shown in Figs. 14, 16, 18, 20, 24, 43, 40; 50, OL, 62,:63, 
GS 1075.08) 705) 71 a7 217.03 000) 2,005. 
The traces of segmentation in the mesodermic area and the 
very conspicuous concrescing folds are interesting features ox 
Figs. 14 and 20. Observe also the curious knob-like ecto- 
dermic thickening at the posterior end of the concresced area 
in Fig. 16, a not infrequent occurrence. Compare also Pl. III, 
Fig. 20 with Pl. V, Fig. 49, where the heart is apparently 
formed for a part of its length. 
The whole process of concrescence, as shown by these em- 
bryos, reminds one of the concrescence of the so-called margin 
of the blastopore in vertebrates. The principal difference is 
that in vertebrates,the ectoderm, mesoderm, and endoderm grow 
over the yolk together, there being no single layered blastoderm 
covering the whole yolk before the germ layers begin to form, 
as is the case with Limulus and the arthropods generally. But 
the presence of this layer of cells can hardly be regarded as a 
serious objection to a comparison of the processes of concres- 
cence in these two great groups of animals. 
In the light of this comparison, the fact that in very rare 
cases, as shown by Ryder in certain fish eggs, the segmenta- 
tion of the mesoderm extends to the margin of the concrescing 
lips of the “blastopore”’ is very significant. It seems to me 
most easily explained as a reversion to a condition like that in 
arthropods. 
There are some other interesting suggestions that arise from 
this comparison. We must not forget that 2 Limulus at any 
vate the growth backwards of this line of concrescence does not 
increase the body in length. It is merely a part of the haemal 
surface, thrown back temporarily upon the neural surface by the 
presence of the yolk. The true increase in length of the axis of 
the body comes from the proliferation of cells in a primitive 
streak that les just in front of where concrescence began, 
It is obvious that an application of these principles of growth 
to vertebrate embryos might clear away some of the difficulties 
