386 E, KAY LANKESTEK. 



tine appears between them without our being able to deter- 

 mine whence it takes its origin ; possibly it is from some 

 of the material of these very cells, but they remain unem- 

 braced by the walls of the alimentary canal so formed, 

 and gradually dwindle by the absorption of their material. 

 In Pisidium, again, we have in the earlier condition a 

 still closer agreement with Lymticeiis, for there a very defi- 

 nitely marked, bilobed gastrula-stomach is formed by in- 

 vagination (see woodcut, fig. 1). But in Pisidium, too, 

 after this epoch, a great change comes over the cells forming 

 the wall of the gastrula-stomach, its cavity becomes con- 

 stricted, narrovved, and contorted, and apparently a new mid- 

 portion of the alimentary canal is formed with separation of 

 nutritive and formative elements from the original gastrula- 

 endoderm-cells. Therefore it seems that the history of the 

 development of the gastrula-stomach into the permanent 

 middle intestine is by no means a simple one. It, in fact, 

 involves the whole question of the part played by the so- 

 called nutritive elements of the original egg-yelk, and we 

 may expect gradations between the developmental processes 

 of a simple unencumbered egg-cell (free from yelk-granules), 

 such as that of the Nematoid Cucullanus (in which I should 

 anticipate that the primitive alimentary canal would be 

 directly enlarged so as to form the adult one), and those of 

 the eggs of Cephalopods and birds, in which the egg-cell is 

 Avell-nigh lost in an excess of superadded nutrient material. 



Wherever these nutritive yelk-elements come in they 

 derange and obscure the usual processes of cell-growth ; in 

 the earliest stages they give a paradoxical twist to the 

 multiplication by fission of the primitive egg-cell, later to the 

 mode of formation of the hypoblast or gastrula-endoderm, 

 and finally, to the mode of development of middle intestine 

 and liver from this last, whilst they may even have something 

 to say to the development of other organs in which their too 

 ready offer of nutritional assistance is accepted. 



The later development of the alimentary canal, the break- 

 ing through of the anus to the exterior, and of the pharynx 

 to the stomach or middle intestine, I have not followed, nor 

 have I observed the development of the liver and absorption 

 of the two masses of pellucid cells which Lereboullet has 

 described, since I have not pursued the embryos to that 

 phase. I may, however, here mention that in the Cephalopod 

 Loligo I have determined, by means of transverse and longi- 

 tudinal sections, that the great mass of " unorganised " yelk 

 enclosed by the embryo is in that animal gradually absorbed, 

 whilst the growth of a pair of diverticula from the alimentary 



