GEOMETRICAL RELATIONS OF CLEAVAGE-FORMS 371 



in sea-urchins it appears at the fourth (Fig. 3); in AmpJiioxus at the 

 third (Fig. 172); in the tunicate Clavclina at the second ( Fig. 177); 

 in Nereis at the first division (Figs. 60, 171). The extent of the in- 

 equality varies in like manner. Taking the third cleavage as a type, 

 we may trace every transition from an equal division (echinoderms, 

 Polygordiiis), through forms in which it is but slightly marked {Avi- 

 phioxns, frog), those in which it is conspicuous {Nereis, Lymniea, poly- 

 clades, Petroinyzon, etc.), to forms such as Clepsine, where the cells of 

 the upper quartet are so minute as to appear Hke mere buds from the 

 four large lower cells (Fig. 172). At the extreme of the series we 

 reach the partial or meroblastic cleavage, such as occurs in the ceph- 

 alopods, in many fishes, and in birds and reptiles. Here the lower 

 hemisphere of the ^gg does not divide at all, or only at a late period, 

 segmentation being confined to a disc-hke region or blastoderm at one 

 pole of the ^gg (Fig. 173). 



Very interesting is the case of the teloblasts or pole-cells character- 

 istic of the development of many annelids and mollusks and found in 

 some arthropods. These remarkable cells are large blastomeres, set 

 aside early in the development, which bud forth smaller cells in reg- 

 ular succession at a fixed point, thus giving rise to long cords of cells 

 (Fig. 175). The teloblasts are especially characteristic of apical 

 growth, such as occurs in the elongation of the body in annelids, and 

 they are closely analogous to the apical cells situated at the growing 

 point in many plants, such as the ferns and stoneworts. 



Still more suggestive is the formation of rudinientaiy cells, arising 

 as minute buds from the larger blastomeres, and, in some cases, appar- 

 ently taking no part in the formation of the embryo (Fig. 174).^ 



We are as far removed from an explanation of unequal division as 

 from that of the rhythm and direction of division. Inequality of divi- 

 sion, like difference of rhythm, is often correlated with inequalities in 

 the distribution of metaplasmic substances — a fact generalized by 

 Balfour in the statement ('80) that the size of the cells formed in 

 cleavage varies inversely to the relative amount of protoplasm in the 

 region of the ^gg from which they arise. Thus, in all telolecithal 

 ova, where the deutoplasm is mainly stored in the lower or vegetative 

 hemisphere, as in many worms, mollusks, and vertebrates, the cells of 

 the upper or protoplasmic hemisphere are smaller than those of the 

 lower, and may be distinguished as micrGineres from the larger viacro- 

 meres of the lower hemisphere. The size-ratio between micromeres 

 and macromeres is on the whole directly proportional to the ratio 

 between protoplasm and deutoplasm. Partial or discoidal cleavage 

 occurs when the mass of deutoplasm is so great as entirely to prevent 

 cleavage in the lower hemisphere. This has been beautifully con- 



1 See Wilson, '98, '99, 2. 



