THE ALIMENTARY CANAL. 



245 



.met. 



in the enteron, and is only gradually absorbed. It still serves as 

 nourishment for the larva, and in Lithobius lasts for fifteen days 

 after hatching. 



Diplopoda. Metschnikoff was the first to point out an important 

 distinction in the formation of the enteron in the two chief divisions 

 of the Myriopoda. In the Chilopoda it arises at the periphery of 

 the yolk (Fig. 124), and thus surrounds the latter, while in the 

 Diplopoda it forms as a tube within the yolk which thus comes to 

 lie outside the gut (Fig. 125). In the Diplopoda also the enteron 

 arises from the cells contained in the yolk, whose origin is as little 

 known here as in the Chilopoda. They collect in a definite region, 

 and become arranged so as to form a large tube extending along the 

 longitudinal axis of the embryo (Fig. 125, md). This tube, which, 

 in the figure taken 

 from Heathcote's 

 drawings, appears as 

 a solid strand, lies, 

 according to the 

 unanimous accounts 

 of Metschnikoff 

 and Heathcote, 

 within the yolk- 

 mass, from which the 

 first -named of these 

 authors was able to 

 remove it entire. 

 The yolk itself thus 

 comes to lie in the 



primary body -cavity, which it completely rills, penetrating, for 

 example, into the spaces left on the separation of the ganglion- 

 rudiments from the ectoderm (Fig. 120 A and B), and even appar- 

 ently (Heathcote) pressing in between the mesoderm-layer and 

 the ectoderm (Fig. 126): The chain of ganglia thus lies within the 

 yolk-mass, and the same is the case with the stomodaeum and proc- 

 todaeum, as is evident from Figs. 125 and 126 B. In later stages 

 the yolk is taken up into the spaces of the pseudocode, and there 

 gradually absorbed (p. 250). 



The presence of yolk in the primary body-cavity has already been mentioned 

 in connection with Moina and Mijsis (Vol. ii., p. 177 . although in these cases it 

 takes place somewhat differently. It may also occur, though on a much smaller 

 scale, in the Insecta, as will be described later. 



Fig. li.J. — Longitudinal section through an embryo of Juliis 

 terrestris on the tenth day of development (after Heath- 

 cote). a, anus ; bg, ventral chain of ganglia ; c, cuticular 

 envelope of the embryo ; d, yolk ; dz, yolk-cells ; kl, cephalic 

 lobe; m, mouth; md, enteron; vies, mesoderm; si, caudal 

 lobe. 



