1889.] Segmentation of the Ovum. 761 
ccelenterates, some sponges, in nematodes, amphibians, etc., it 
is less marked and appears later. 
In most cases the entodermic cells are very decidedly larger 
and less numerous than those of the ectoderm. This distinction 
is obviously necessary on account of the mutual relations of the 
two primitive layers. The ectoderm has to grow around the 
entoderm, which it can do only by acquiring a greater superficial 
extension ; this the ectoderm accomplishes by dividing very 
quickly at first into small cells. After the entoderm is fully en- 
veloped it may then continue to grow until its superficies is much 
greater than that of the outer layer, within which, however, it 
still finds room by forming numerous folds ; thus is gradually 
reached the condition in the higher adult animals, where the in- 
testine sometimes has an enormous surface, but is, nevertheless, 
contained in body-walls covered by ectoderm presenting much 
less surface. It is, therefore, only during the early stages of seg- 
mentation that we find the entoderm expanding more slowly than 
the ectoderm. 
The terms holoblastic and meroblastic are applied to ova accord- 
ing to their manner of segmentation. The first is employed for 
those ova in which there is either very little or only a moderate 
amount of yolk, so that the whole of the ovum splits up into 
distinct masses (cells) which enter into the composition of the em- 
bryo. The second designates ova with 
very large amount of yolk, so that while th 
protoplasm from which the ectoderm arise 
divides rapidly into distinct cells, the 
dermal portion merely dev^elops nuclei at 
first, with the result that while one portion 
of the t^^ is "segmenting," another portion 
(the entodermal) remains unsegmented, so 
far as the external appearances are concerned. 
Eggs, then, with much yolk undergo the Biastuia stage in the 
so-called partial segmentation ; hence the ^J^^IX"'^"rc/aL;f ^tfter 
adjective meroblastic. s'eienka. 
Whatever the exact mode of segmentation, there results always 
the same type of organization, to which Minot has applied the 
