98 UNEQUAL SEGMENTATION. 



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

 smaller segments always become epi blast and of the larger segments hypo- 

 blast. 



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 

 segmentation. 



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 envelop 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 sub- 

 regular 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 



1 Van Beneden, " D^veloppement embryonnaire des Mammiftres." Bull, de 

 FAcad. Belgique, 1874. 



