ARACIINIDA. 



465 



led to this condition of tlie excluded ovum, have been ascertained to 

 be due to the i^ttraction and assimilation by the primary gei-m-cell 

 and its progeny of a small proportion of the yolk, which is thus seen 

 to consist of a gcrm-yolk and a food-yolk. The subsequent processes, 

 up to the complete formation of the young spider, have been de- 

 scribed and figured by the accurate and industrious Herold.* 



The germ-mass consists of derivative germ-cells, like minute 

 opaque whitish granules of smaller diameter than those of the vitellus ; 

 in some species Ilerold observed wiiat he believed to be several 

 germ-spots on different parts of the superficies of the yolk, which 

 rapidly coalesced into one body. Development commences by expan- 

 sion of the circumference of the germ-mass, which, as it expands, 

 covers the yolk with a semi-transparent thin layer, the basis of the 

 future integument. Ilerold next describes the granules of the germ- 

 mass as being decomposed into almost imperceptible molecules, in 

 which we may recognise the ordinary result of the fissiparous pro- 

 perty of its constituent nucleated cells : their powers of assimilation 

 are at the same time manifested by the changes which they effect in 

 the albumen, at tlie expense of which they seem, in the first instance, 

 to increase their numbers, and diffuse themselves over the surface 

 of the vitellus. This covering of the yolk Herold calls " coliqua- 

 mentum." He observes, that the original position of the germ-spot 

 is indicated by a clear, transparent point (hyaline ?) ; that tliis point 

 becomes thickened, pearly, and opaque, so as to conceal the subjacent 

 vitelline cells. He calls this the nucleus-of the germ or the catribium 

 {fig. 174, «). A similar change progressively extends over the col- 

 liquamentum ; and, when one-fourth of the circumference of the yolk 

 is thus covered, the opaque layer has taken on a definite form. 



174 



175 



176 



Development of Spider. 



resembling the figure 8 (^fig. 175), the small and anterior division (a) 

 being the base of the future head, the posterior and larger one (6), 

 of the thorax. A fissure is next observed to divide the cephalic 

 (^fig. 176, a) from the thoracic portion (6), the two parts being 

 distinct at this period, and determining the essential nature of the 



* CCXLI De gencratione Aranearum in Ovo, 1824. 



n II 



