102 UNEQUAL SEGMENTATION. 



The ovum contains a large amount of food-yolk, and the protoplasm is 

 aggregated at the formative pole, adjoining which are placed the polar 

 bodies. An equatorial and a vertical furrow (fig. 43 A), the former near 

 the upper pole, appear simultaneously, and divide the ovum into three 

 segments, two small, each with a protoplasmic pole, and one large en- 

 tirely formed of yolk material. One of the two small segments next com- 

 pletely fuses with the large segment (fig. 43 B), and after the fusion is com- 

 plete, a triple segmentation of the large segment takes place as at the first 

 division, and at the same time the single small segment divides into two. In 

 this way four partially protoplasmic segments and one yolk segment are 

 formed (fig. 43 C). One of the small segments again fuses with the large 

 segment, so that the number of segments becomes again reduced to four, 

 three small and one large. The protoplasmic ends of these segments are turned 

 towards each other, and where they meet four very small cells become budded 

 off, one from each segment (fig. 43 D). Four small cells are again budded 

 off twice in succession, while the original small cells remain passive, so that 

 there come to be twelve small and four large cells. In later stages the four 

 first-formed small cells give rise to still smaller cells and then the next- 

 formed do the same. The large cells continue also to give rise to small 

 ones, and finally, by a continuous process of division, and fresh budding of 

 small cells from large cells, a cap of small ceils becomes formed covering 

 the four large cells which have in the meantime pressed themselves together 

 (fig. 43 E). A segmentation cavity of not inconsiderable dimensions be- 

 comes established between this cap of small cells and the large cells. 



Many eggs, such as those of the Myriapods 1 , present an irregular seg- 

 mentation ; but the segmentation is hardly unequal in the sense in which I 

 have been using the term. Such cases should perhaps be placed in the first 

 rather than in the present category. 



The type of unequal segmentation is on the whole the most widely 

 distributed in the animal kingdom. There is hardly a group without ex- 

 amples of it. 



It occurrs in Porifera, Hydrozoa, Actinozoa and Ctenophora. Amongst 

 the Ctenophora this segmentation is of the most typical kind. Four equal 

 segments are first formed in the two first periods. In the third period a 

 circumferential furrow separates four smaller from four larger segments. 



This type is also widely distributed amongst the unsegmented (Gephyrea, 

 Turbellaria), as well as the segmented Vermes, and is typical for the 

 Rotifera. It appears to be very rare in Echinoderms (Echinaster Sarsii). 

 It is not uncommon in early stages of the segmentation of the lower 

 Crustacea. 



For Mollusca (except Cephalopoda) it is typical. Amongst the Ascidia 

 it occurs in several forms (Salpa, Molgula) and amongst the Craniata it 

 is typical in the Cyclostomata, Amphibia, and some Ganoids, e.g. Acci- 

 Penser. 



1 Metschnikoff, Zeitschrift f. wiss. Zoologie, 1874. 



