: 
1892.] Embryology. 1045 
blastopore). Between these two limits it takes the course of a reversed 
letter S. The anterior part perforates the medullary canal and is 
dorsal; this is continuous round the end of the tail with a ventral 
part, which extends forward along the ventral side of the tail as far 
as the yolk-stalk. Along this it passes to continue backward along the 
yolk as the slit-like, non-embryonic part of the blastopore, which 
passes behind into the more dilated and posterior part of the so-called 
yolk-blastopore. In the author’s view the blastoderm grows uniformly 
over the yolk at all points of its circumference. The notch of the 
embryonic rim represents the anterior end of the blastopore and the 
blastopore does at one time or another perforate the whole length of 
the medullary plate; anteriorly it keeps closing up as the embryonic 
rim grows backwards, so that it is never present in this region as more 
than a notch. Yet there is no ‘‘concrescence”—“ to talk about concres- 
cence and fusion of two halves is merely obscuring the real question 
and seeking to explain a process of growth by a phrase which has no 
satisfactory meaning.” 
The anus is formed within the area of the blastopore; is actually a 
part of the blastopore in some vertebrates; not so the mouth. 
In Scyllium canicula the mouth is at first a longitudinal row of 
dots between the mandibular arches. These pores become connected 
to form a longitudinal slit extending forward into the rudiment of the 
pituitary body. Later the mouth shortens and widens to the adult 
form. Its slit-like form may be favorable to the view that it is derived 
the anterior part of the blastopore, “ though I admit that this 
does not constitute a very powerful argument.” 
The mandibular, hyoid and branchial arches are very much bent 
backward, which suggests that there has been not only a cranial but a 
cephalic flexure affecting brain, notochord and arches as well. Before 
such a flexure the mouth was a vertical slit looking forward or even 
extending onto the dorsal surface. 
Regarding the metamerism of the head the author thinks that the 
number of primitive somites in any region of the head differs in closely 
allied genera. Hence the adult segmentation, constant throughout 
the vertebrates, has little value in determining the primitive metamer- 
ism. “We may, I think, even go farther and say that the adult 
~ Arrangement of nerves and branchial arches, ete., characteristic of the 
Vertebrate head, must have arisen subsequently to the disappearance 
of the primitive segmentation.” The first or premandibular somite of 
— , arising from a mass at first connected with the ectoderm, 
With the notochord and with the anterior end of the gut, thus presents 
