VIII 



AKTHEOPODA 



223 



areas cause the egg, when it is newly laid, to appear as if it had 

 already undergone segmentation, and the whole structure of the egg 

 at this time irresistibly reminds one of the segmented egg of Astacus 

 with its primary yolk pyramids. Nevertheless, the resemblance 

 between the two eggs is purely illusory ; for the egg of the spider 

 when newly laid contains only one nucleus, and the polygonal areas 

 seen in the periplasm are due to the pressing of the soft periplasm 

 against the resisting yolk columns as the egg is squeezed in passing 

 down through the oviduct. 



When the nucleus divides into two the yolk columns become 

 massed together into two rosette-like bundles, with one nucleus in 

 the centre of each bundle ; and a small blastocoele appears at the 

 centre of the egg. The same process is repeated when the next 

 division takes place, and so there are as many yolk rosettes as nuclei. 

 In consequence of this rearrangement of the yolk columns they shift 

 with relation to the 

 polygonal areas of 

 the periplasm, and 

 these latter no longer 

 form caps to them. 

 As the nuclei in- 

 crease in number the 

 segmentation cavity 

 enlarges, and the 

 nuclei gradually 

 travel towards the 



Surface of the e^g 



When the number 



^f-,^ ,^1 Vvr> ir>4-t-r>-ii->ns1 A. staije in which the primitive streak and the primitive cumulus are 

 Ol nUClcl IlabclUlcllIlcll , , , . , ., .. , , ... 



connected. B, stage in which the connection between the two is broken 



oU, all reaCD the SUl'- and the primitive cumulus is migrating backwards (V). (After Kautsch.) 



face ; but the multi- 

 plication of the nuclei still continues, and when 100 nuclei have been 

 formed the periplasm becomes completely separated from the yolk and 

 transformed into a blastoderm of rounded cells. 



The blastocoele seems by this time to have disappeared. As the 

 blastodermic cells multiply they become more flattened, except at 

 one spot, which Kishinouye names the primitive ventral thickening 

 and which we may term the primitive streak. Here the blastoderm 

 cells remain rounded; they proliferate cells into the interior of the 

 egg, and thus initiate the gastrula stage and the formation of layers. 

 The primitive streak becomes elongated, and at one end of it another 

 thickening appears, caused also by the cells of the blastoderm retaining 

 a rounded shape and proliferating inwards. 



What the meaning of this secondary thickening is Kishinouye 

 could not ascertain, because he could not 'determine whether the side 

 of the primary thickening, where the secondary thickening appeared, 

 was anterior or posterior ; it appears probable, from what occurs in 

 the development of other Arachnida, that this secondary thickening 



Fiu. 170. Surface views of the developing egg of Agelena 

 labyrinthica, showing the primitive streak and the primitive 

 cumulus. (After Kautsch. ) 



