34 
GEORGE BROOK. 
one end to the other. Free nuclei are also seen both in the 
yolk and on the floor of the segmentation cavity. Although 
earlier phases of the hypoblast were not observed in Motella, 
it seems impossible that such large and well-defined cells could 
be the result of invagination from the cells of the archiblast, 
when the cells of the latter are scarcely distinguishable under 
a magnifying power of 100 diameters. When, however, we 
have the data arrived at from a study of Trachinus to work 
upon, there is no difficulty in accounting for their origin. 
If, now, my figures, which are carefully copied from actual 
sections (excepting figure 7), be compared with those of 
van Bambeke, Klein, and van Beneden, it will be seen that 
our observations agree very closely. Indeed, van Beneden’s 
fig. 9 (loc. cit.), which also represents a pelagic egg, shows the 
identical process at work which I have described for Tra- 
chinus. The question then arises, is the hypoblast formed 
by a true process of invagination ? It is quite true that 
the rim grows from the margin inwards because the cells 
from the periblast commence at the margin close to the over- 
lapping epidermic layer and are filled up from within. Is not 
something more than this meant by invagination ? I take it 
that invagination in the true sense means an ingrowth or an 
infolding of a layer already existing, the archiblast. If this 
be so, there is no true invagination in such pelagic ova as those 
here described, and the hypoblast is not derived from the 
archiblast at all, but from the periblast and the yolk by a 
process of segregation. 
List of Papers referred to. 
1. Agassiz and Whitman. — “Ou the Development of Some Pelagic 
Fish Eggs,” ‘ Proc. Amer. Acad, of Arts and Sciences,’ vol. xx, 
1884. 
2. C. K. Hoffmann. — “ Zur Ontogenie der Knochenfische,” ‘ Natuurk. 
verb. d. Koninkl. Akad. Amsterdam,’ vol. xxi, 1881. 
3. E. van Beneden. — “ A Contribution to the History of the Embryonic 
Development of Teleosteaus,” ‘ Quarterly Journ. of Microsc. Sci.,’ 
vol. xviii, N.S., 1878. 
