122 COMPARATIVE ANATOMY 



former divides, and forms, with its attraction sphere, also 

 divided, an achromatic figure of the normal type. The skein 

 in each of the germ-nuclei now resolves itself into two 

 chromosomata having the form of bent rods, and the four 

 chromosomata become attached to the equator of the spindle 

 of the achromatic figure. Each is split into two longitudinally, 

 and a half of each passes to opposite poles of the spindle. 

 Division of the body of the ovum then takes place in the 

 normal manner, and two embryo-cells or blastomeres are 

 formed, each of which contains four chromosomata, half 

 derived from the male, half from the female nucleus. The 

 processes of maturation of the ovum, development of the 

 spermatozoa, and fertilisation are essentially the same in the 

 frog as those described for Ascaris, though differing in details 

 which appear to be unimportant. The number of chromo- 

 somata in the cells of the frog is twenty-four. These are 

 reduced in the reducing divisions of ova and spermatozoa to 

 twelve, and the number is restored on union of the sperm 

 and egg-nuclei to twenty-four. The spermatozoon of the 

 frog, however, is very different to that of Ascaris, consist- 

 ing of (i) a head, (2) a minute segment immediately behind 

 the head, which appears to consist of archoplasm with a 

 contained centrosome, (3) a whip-lash-like tail or flagellum, 

 by whose movements the spermatozoon is propelled through 

 liquid. The head consists entirely of nuclear matter, except 

 for a very thin layer of cytoplasm, which probably forms an 

 investment to it. 



The ovum of the frog, as has been said, is abundantly 

 provided with food-yolk, stored up chiefly in one hemisphere 

 of the cell. The first segmentation following upon fertilisa- 

 tion divides the ovum into two equal halves, or blastomeres. 

 A pause follows, and the ovum divides again, the plane of 

 the second division being at right angles to the first. Thus 

 four blastomeres are formed, each consisting of a smaller 

 upper pigmented protoplasmic part, and a lower colourless part 

 filled with deutoplasm. The third segmentation is described 

 as equatorial, but it does not pass through the equator of 

 the spherical ovum, but is placed nearer the pigmented pole. 

 It nearly completely cuts off four pigmented upper blastomeres 

 from four lower heavily-yolked blastomeres. Hitherto the 

 planes of division have passed right through the ovum, 



Digitized by Microsoft® 



