104 ON THE ORIGIN AND [CHAP. 



cleavage of the yolk into two segments (PL 6, Fig. 2), 

 immediately followed by the cleavage of these into 

 other two, so that the vitellus is now composed of 

 four cleavage spheres (PI. 6, Fig. 3)." These spheres 

 again divide (PL 6, Fig. 4) and subdivide, thus at 

 length forming minute cells, of which the body of the 

 embryo is built up. 



In PL 6, Figs. 5 9 represent the corresponding 

 stages in the development of a small parasitic worm 

 the Filaria mustelarum as given by Van Beneden. 1 

 The first process is that within the egg, which repre- 

 sents, so to say, the encysted condition of Mago- 

 sphsera, the yolk divides itself into two balls (PL 6, 

 Fig. 6), then into four, eight, and so on, the cells 

 thus constituted finally forming the young worm. 

 I have myself observed the same stages in the eggs 

 of the very remarkable and abnormal Sphczmlaria 

 bombi? 



Among the Echinoderms M. Derbes thus describes 

 the first stages (PL 6, Figs. 10 13) in the develop- 

 ment of the egg of an Echinus (Echinus esculentus) : 

 " Le jaune commence a se segmenter, d'abord en 

 deux, puis en quatre et ainsi de suite, chacune des 

 nouvelles cellules se partageant a son tour en deux." 3 

 Sars has observed the same thing in the star-fish. 4 



In the Rotatoria, as shown by Huxley in Lacinu- 

 laria, 5 and by Williamson in Melicerta, 6 the yolk is at 



1 Mem. sur les Vers Intestinaux, 1858. 



2 Natural History Review, 1861, p. 44. 



3 Ann. des Sci. Nat. 1847, p. 90. 



4 Fauna littoralis Norvegice, pi. viii, 



6 Trans, of the Microsc. Soc. of London, 1851. 

 6 Quarterly Journal of Microsc. Science, 1853. 



