32 
GEORGE BROOK. 
takes no part in the formation of the hypoblast. Hoffmann 
asserts that the invaginate layer of the ring and embryonic 
shield is split off from the archiblast as a primary ento- 
derm, and that this afterwards is differentiated into the meso- 
derm (several cells deep) and the secondary entoderm 
(only one cell deep). Henneguy 4 also states that in the 
trout the invagination is caused by an infolding of the 
segmented disc upon itself, and that, as the invagination pro- 
gresses, the lower invaginated portion remains quite separate 
from the original upper portion, and that indeed he can demon- 
strate a space between them. On the other hand, van 
Bambeke (11), Klein (8), Kupffer (7), and van Beneden (3), 
agree that the hypoblast is derived from the periblast, while 
Balfour (10), Ryder (6), Kingsley and Conn (9), and others, 
admit that the periblast plays a more or less important part in 
the structure of hypoblastic tissues. Haeckel (5) failed to 
recognise the periblast altogether, and his diagrams of the 
invagination of the hypoblast can scarcely be true for any 
Teleostean. The eggs studied by Haeckel were pelagic and 
supposed to belong to a species of Motella; but I think there 
must be some mistake here, as I have studied the development of 
Motella mustela and can confidently assert that the whole 
process of development is different from that given by Haeckel. 
With a view to studying the process of invagination in 
Trachinus, I have preserved eggs at half-hourly intervals from 
a little before the appearance of the blastodermic rim until 
this was quite well-defined all round the disc, a period of deve- 
lopment occupying four hours at a temperature of about 
62° Fahrenheit. The eggs were prepared by the picrosul- 
phuric acid method and stained with cochineal. It is on 
sections made from these eggs that my conclusions are based. 
Figure 2 represents a section of one of the earlier of these 
stages, in which the periblast is seen to be collected around 
the margin of the disc and to have pushed itself some little 
way underneath ; but there does not yet appear to be a layer 
of periblast on the floor of the segmentation cavity. The 
epidermic layer of the epiblast is, however, well differen- 
