282 



INVEKTEBKATA 



CHAP. 



called the formation of primary yolk pyramids, and which he 

 interpreted as an incomplete segmentation of the egg. Heymons 

 endeavours to give the same interpretation here, but there are great 

 difficulties in the way. Thus, in the egg of Astacus each primary 

 pyramid contains a nucleus in its outer portion, where it is most 

 sharply marked off from its neighbours, but in Scolopendra all the 



nuclei are contained in the central un- 

 segmented yolk, and it is more than 

 doubtful if they correspond either in 

 number or arrangement with the 

 pyramids. 



The blastoderm in Astacus is derived 

 from the outermost portions of the yolk 

 pyramids containing the nuclei, this 

 portion being cut off from the rest of the 

 yolk, but in Scolopendra the blastoderm 

 originates as in Insecta, through the 

 migration of nuclei from the centre to 

 the periphery of the egg. These nuclei 

 are surrounded by bodies of cytoplasm 

 forming " free cells," as they may be 

 termed, and they migrate outwards 

 between the adjacent pyramids. It is 

 clear, therefore, that the pyramids in 

 Scolopendra have no relation to the 

 division of the egg into blastomeres, but 

 -prod are a phenomenon of the same kind as 

 we met with in the egg of the Spider, 

 the peculiar yolk-rosettes of which are 

 quite unconnected with the true seg- 

 mentation. 



If we eliminate the yolk pyramids 

 the formation of the layers is almost 



mn, mandible ; ma;i, ft^t maxilla ; iru. 2 , 

 second maxilla ; mxp, maxillipede 

 (poison-claw) ; o, mouth ; pr.ant, pre- 

 antenna;proc, opening of proctodaeum. 



FIG. 225. Ventral view of the 

 embryonic area in a developing 

 egg of Scolopendra cengulata. 

 (After Heymons. ) 



ant, antenna ; app, appendages of -,,.-,. 7 i r 



the body -, c .i, caudal lobe; fob, labrum ; identical m bcolopendra and Lepisma. 



The endoderm is budded off from a thick- 

 ened area of the blastoderm termed by 

 Heymons the "cumulus primitivus,"but 

 which we might more appropriately term 

 the primitive streak; the cells which ultimately form the endo- 

 derinal epithelium and those that form yolk cells are indistinguishable 

 from one another; the mesodermic bands arise from the hinder 

 border of the cumulus and extend forwards, and their extent defines 

 the embryonic area as in Insecta. 



(&) The division of the body into segments and the outgrowth 

 of the appendages therefrom occurs as in Insecta. There is an 

 intercalary segment which, however, develops no appendages ; there 

 are three jaw segments, behind which comes the segment bearing 

 the powerful poison claws characteristic of Chilopoda. But in 



