208 ¥F. M. BALFOUR. 
duced. Many vertebrates have in the first stages of their 
development a number of secondary characters which are 
due to the presence of food material in the ovum ; the present 
essay is mainly an attempt to indicate how those secondary 
characters arose and to trace their gradual development. At 
the same time certain important ancestral characters of the 
early phases of the development of vertebrates, especially 
with reference to the formation of the hypoblast and meso- 
blast, are pointed out and their meaning discussed. 
There are three orders of vertebrates of which no men- 
tion has been made, viz., the Mammals, the Osseous fishes, 
and the Reptiles. The first of these have been passed over 
because the accounts of their development are not suf- 
ficiently satisfactory, though as far as can be gathered 
from Bischoff’s account of the dog and rabbit there would 
be no difficulty in showing their relations with other verte- 
brates. 
We also require further investigations on Osseous fishes, 
but it seems probable that they develop in nearly the same 
manner as the Klasmobranchs. 
With reference to Reptiles we have no satisfactory investi- 
gations. 
Amphioxus is the vertebrate whose mode of development 
in its earliest stages is simplest, and the modes of develop- 
ment of other vertebrates are to be looked upon as 
modifications of this due to the presence of food material in 
their ova. It is not necessary to conclude from this that 
Amphioxus was the ancestor of our present vertebrates, but 
merely that the earliest stages of development of this verte- 
brate ancestor were similar to those of Amphioxus. 
The ovum of Amphioxus contains very little food material 
andits segmentation is quite uniform. The result of segmen- 
tation is a vesicle whose wall is formed of a single layer 
of cells. These are all of the same character, and the cavity 
of the vesicle called the segmentation cavity is of consider- 
able size. A section of the embryo,as we may now call the 
oyum, is represented in Plate X, fig. a 1. 
The first change which occurs is the pushing in of one 
half of the wall of the vesicle towards the opposite half. 
At the same time by the narrowing of its mouth the hollow 
hemisphere so formed becomes again a vesicle.! 
'T have been able to make at Naples observations which confirm the 
account of the invagination of Amphioxus as given by Kowalevsky, though 
my observations are not nearly so complete as those of the Russian 
naturalist. 
