82 Mr. E. Ray Lankester's Zoological Observations 



villi into the growing egg. This cellular lining of the capsule 

 grows very rapidly ; and its cells are continually being absorbed 

 or fused into the vitellus, whence the increase of this in size. 

 Some of the cells retain their form and are to be found floating 

 in the complex vitellus thus built up. 



On attaining full size, the egg, having lost entirely/ its 

 large germinal vesicle, loses all the plications or basketwork 

 of its vitelline surface, and escapes from its capsule, which 

 remains on the branched ovary and undergoes a yellow dege- 

 neration. Passing as a free ovoid homogeneous mass of com- 

 plex yelk (protoplasm and deutoplasm, Van Beneden, com- 

 bined) into the oviduct, the egg is fertilized ; and then at one 

 pole a segregation of plastic yelk, or a germinal patch, occurs 

 in the form of a thin disk or cap. This exhibits subsequently 

 a faint nucleus and commences to divide into two, four, eight, 

 &c. arese, marked out by intercrossing grooves. In some 

 minor respects my observations differ from Kolliker's, who 

 appears to have represented the segmentation as more regular 

 than it is, and the resulting cells as becoming detached, which 

 they do not. 



When this superficial layer of blastodermic cells has spread 

 over an area relatively as large as would be inclosed by the cir- 

 cumference of a half-crown drawn round the pole of a large hen's 

 egg, an exceedingly remarkable fact presents itself, which has 

 not been observed before, and which has great importance in re- 

 gard to the various theories as to the origin of the " mesoderm," 

 or a portion of that layer. Outside the primitive segmentation- 

 area (fig. 1 p) , and quite unconnected with it, appears a ring of 

 very large pellucid nuclei, seven or eight in number (fig. 1 d) ; 

 they increase in number, and a second, third, and fourth ring of 

 such large nuclei arise, till at last they spread over the whole 



Fi-. 1. 



egg. Meanwhile the cells of the segmentation-area spread by 



