80 UNEQUAL SEGMENTATION. 



followed by an equatorial furrow. The general laws which were 

 stated with reference to the velocity of segmentation and the size 

 of the resulting segments are well exemplified in the case of the 

 Frog's ovum. 



The majority of the smaller segments in the segmented Frog's 

 ovum are destined to form into the epiblast, and the larger 

 segments become hypoblast and mesoblast. 



With a few exceptions (the Rabbit, Lymrueus, etc.) the majority of the 

 smaller segments always become epiblast and of the larger segments hypoblast. 



The Frog's ovum serves as a good medium type for unequally 

 segmenting ova. There are many cases however in which a regular 

 segmentation is far more closely approached, and others in which 

 it is less so. 



One familiar instance in which a regular segmentation is nearly 

 approached is afforded by the Rabbit's ovum, which has indeed 

 usually been regarded as offering an example of a regular seg- 

 mentation. 



The ovum of the Rabbit 1 becomes first divided into two sub-equal 

 spheres. The larger and more transparent of the two may, from 

 its eventual fate, be called the epiblastic sphere, and the other the 

 hypoblastic. The two spheres are divided into four, and then by 

 an equatorial furrow into eight four epiblastic and four hypoblastic. 

 One of the latter assumes a central position. The four epiblastic 

 spheres now divide before the four hypoblastic. There is thus 

 introduced a stage with twelve spheres. It is followed by one with 

 sixteen, and that by one with twenty-four. During the stages 

 with sixteen spheres and onwards the epiblastic spheres gradually 

 envelope the hypoblastic, which remain exposed on the surface at 

 one point only. There is no segmentation cavity. 



In Pedicellina, one of the entoproctous Polyzoa, there is a 

 subregular segmentation, where however the two primary spheres 

 can be distinguished much in the same way as in the case of the 

 Rabbit. 



A very characteristic type of unequal segmentation is that 

 presented by the majority of Gasteropods and Pteropods and probably 

 also of some Lamellibranchiata. It is also found in some Turbellarians, 

 in Bonellia, some Annelids, etc. In many instances it offers a good 

 example of the type where in the course of segmentation the 

 protoplasm becomes aggregated at one pole of the ovum, or of its 

 segments, to become separated off as a clear sphere. 



The first four segments formed by two vertical furrows at right 

 angles are equal, but from these there are budded off four smaller 

 segments, which in subsequent stages divide rapidly, receiving 

 however, a continual accession of segments budded off from the larger 

 spheres. The four larger spheres remain conspicuous till near the 



1 Van Ben den, "Developpement cmbryouuaire des Marnmiferes." Bull, dc VAcatl. 



