1889.] Segmentation of the Ovum. 763 
divided into discrete cells, but remains one mass; this is the case 
in elasmobranchs and the amniota, but in the highest amniota 
(Placentaiia) the yolk is lost and the entoderm is again represen- 
ted by a single layer of cells (Fig. 13). 
It seems to me evident that the first step of development in the 
segmenting ovtini is the differentiation of the tzuo germ-layers, ecto- 
derm and entoderm, resulting in the diaderm stage. Diaderm is 
a term preferable to blastula, because the latter is applicable 
strictly only to a special larval form, while the former is a general 
term which refers to the essential differentiation at this stage. It 
is important to remark that the two layers are distinct in the 
diaderm or blastula stage ; it is often erroneously affirmed that 
the blastula consists of a uniform layer of cells, part of which 
subsequently becomes the entoderm. 
The segmentation cavity comprises the whole space between 
the entoderm and ectoderm ; it is very early invaded by cells pro- 
duced from the two primitive germ-layers. These cells are in 
vertebrates of many kinds, and enter the segmentation cavity at 
various periods. It is customary to group the cells which enter 
early into this cavity under the common name of mesoderm, and 
to consider them as a third and distinct germ-layer. For con- 
venience we may adopt this custom, for, to a certain extent, the 
mesoderm of authors is a separate germ-layer, but it by no means 
includes all the tissues which occupy the space between the two 
primitive germ-layers. As the space between the entoderm and 
ectoderm is always homologous with itself it follows that the 
entire room between the epithelium (entoderm) of the digestive 
tratt and its appendages on the one side, and the epiderm on 
the other, is homologous with the segmentation cavity. 
The mesoderm of authors comprises three tissues: (i), free 
wandering cells {mesamcvboids) ; (2), embryonic connective tissue 
or cells connected together by processes {mesenchyms) ; (3), epi- 
thelium, which forms two or more separate sacks. The origin of 
the mesoderm and the relations of the three tissues it contains do 
not fall within the scope of this article. 
The Gastrula theory. — In invertebrates with holoblastic ova the 
blastula passes into a stage known as the Gastrula. Gastrula is, 
