588 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
The first rudiment of the mouth actually extends into the rudiment 
of the pituitary body ; all the arches are diverted backwards, and the 
mandibular most of all ; this appears to be due to that flexure which is 
ordinarily called cranial, but would better be called cephalic. 
Three views have been put forward as to the origin of the peripheral 
nerves ; that which the late Prof. Balfour preferred, but which he 
rejected on the ground of want of supporting evidence, was the doctrine 
of Hensen that the rudiments of the nerve-fibres are present from the 
beginning as persistent remains of the primitive connections between 
the incompletely separated cells of the segmented ovum. Mr. Sedgwick 
is now able to supply the needed evidence, for he can certify that there 
is not a complete separation of the cells of the segmented ovum. 
Subsequent to cleavage the cells of the young embryo are connected by 
delicate processes, often extremely fine ; these unite together into networks 
below the epithelial arrangement of the protoplasm which is charac- 
teristic of the surfaces. This network is often of a very loose mesh and its 
fibres are always delicate ; and it is, doubtless, often torn and destroyed 
by the preserving processes to which the embryo has to be subjected. 
But Mr. Sedgwick assures us, delicate as it is, there can be no doubt of 
its existence in Vertebrate embryos; and there can be no reasonable 
doubt that it is derived from the processes and strands left between the 
cells, as a result of the incomplete cleavage of the ovum. However, it 
still remains to be shown that the nerve-fibres are derived from it. 
Utero-gestation in Trygon Bleekeri.* — Dr. A. Alcock reminds us 
that one of the most interesting discoveries made by the Indian survey 
steamer ‘ Investigator,’ is that there are certain Elasmobranch fishes 
in which the female developes during pregnancy a vast system of uterine 
glands that secrete a nutrient fluid or uterine milk for the nurture of 
the developing embryo. Having had the opportunity of examining two 
pregnant specimens of Trygon Bleelceri , he tells us that he found the 
single uterus to contain a single naked foetus unattached structurally 
to the mother ; the uterine mucous membrane was produced into long 
villi, which consisted almost exclusively of blood-vessels and glands. 
The viscid, turbid, or milky secretion of the latter was found free in 
the uterine cavity. The secretion was further observed unchanged in 
the spiracles and in coagula filling the duodenum and anterior part 
of the spiral gut of the foetus. 
Embryonic History of Pteroplatsea micrura.j — Dr. A. Alcock, who 
with Prof. Wood-Mason has already shown that in Pteroplatsea micrura 
the ovum is retained within the uterus and that the uterine mucous 
membrane is furnished with nursing filaments or trophonemata, which 
secrete a “ milk ” that supplies the embryo with nutriment, now gives 
some interesting observations on the embryonic history of this fish. 
The embryo when 29 mm. long has a remarkable generalized shark- 
like form. The broad cord by which it is attached to the large yolk-sac 
is traversed by channels which do not contain blood- cells but only 
spherules of yolk. The total volume of the gill-filaments was not less 
than one-third that of the entire embryo. Each filament is nothing 
* Ann. and Ma". Nat. Hist., ix. (1892) pp. 417-27 (1 pi.). 
f Op. cit„, x. (1892) pp. 1-8 (1 pi.). 
