SOME TEACHINGS OF DEVELOPMENT. 
205 
which from the first have scarcely any store of nutriment, dis- 
appears. The difference is one of degree only and not a difference 
in the essential nature of the ova ; this view is confirmed by the 
existence of every shade of transition between them, as well as by 
observing that the difference obtains in the ova of animals which 
are otherwise closely allied. 
Further, in the ovum of all animals there follows immediately 
on fertilisation the successive division or cleavage of the single 
cell into first two, then four, and so on until an indefinite number 
of cells is formed which arrange themselves around a central space 
— the cleavage-cavity. This, in the majority of cases, is distinct 
enough, being occupied merely by clear fluid and enclosed by a 
single layer of cells; but in some instances it is partially or 
wholly filled up by the excess of food-material and its immediately 
attendant protoplasm, which may or may not, according to its 
relative bulk, have participated in the active changes which have 
been occurring in the rest of the protoplasm, and of which the 
cleavage is the result. To revert to our military comparison it 
is as if the two armies had broken up into detachments and 
arranged themselves so as to enclose circular areas of ground ; 
in the one case the detachments unencumbered and active on all 
sides, with the enclosed area left clear ; in the other case the 
materiel^ with the camp followers, packed away in the centre 
and partly occupying one side of the space, whilst the least 
encumbered detachments are chiefly accumulated on the side 
where most activity is required. 
Bearing in mind this difference, and regarding it as accidental, 
we may take the completion of the cleavage process, as far as 
the arrangement of the resulting cells around a central area in 
the manner described, as the first stage in the development of 
all animals. It may be termed the stage of the unilaminar 
blastoderm^ or the hlasto sphere. For the name blastoderm has 
long been applied to the cellular membranes of wdiich the devel- 
oping ovum is formed at early periods ; and the term blasto- 
sphere may be used, because at this stage a typical ovum, i.e. 
one wuth a minimal amount of nutritive material, consists as we 
have seen of but a single uniform layer of cells forming the wall 
of a hollow sphere. 
We meet with this typical condition of the first stage in Gas- 
trophy seraa (according to Haeckel), in Phallusia, and in Am- 
phioxus and other animals. But even in those ova w^hich con- 
tain only a slight excess of pabulum we find that there are 
differences observable in the cells which form the wall of the 
blastosphere, for the food-material becomes chiefly collected at 
one part of the layer, rendering the cells at this part more 
granular and larger than the others. Tliis difference may be so 
