226 GASTEROPODA AND PTEROPODA. 



the various species differ immensely as to the amount of food- 

 yolk, considerable differences obtain in the mode of formation of 

 the layers and of the alimentary tract. 



The spheres at a very early stage of segmentation 1 become 

 divided into two categories, one of them destined to give rise 

 mainly to the hypoblast, the other mainly to the epiblast. Ac- 

 cording as there is much or little food-yolk the hypoblast spheres 

 are either very bulky or the reverse. In all cases the epiblast 

 cells lie at one pole, which may be called the formative pole, 

 and the hypoblast cells at the opposite pole. When the bulk of 

 the food-yolk is very great, the number of hypoblast spheres is 

 small. Thus in Aplysia there are only two such spheres. In other 

 cases, where there is but little food-yolk, they may be nearly 

 as numerous as the epiblast cells. In all these cases, however, 

 as was first shewn by Lankester and Selenka, a gastrula becomes 

 formed either by normal invagination as in the case of Paludina 

 (fig. 107), or by epibole as in Nassa mutabilis (fig. 105). In both 

 cases the hypoblast becomes completely enclosed by the epiblast. 

 TJie blastopore is always situated opposite the original formative 

 pole. In the large majority of cases (i.e. Marine Gasteropoda, 

 Heteropoda, and Pteropoda) the blastopore becomes gradually 

 narrowed to a circular opening which eventually occupies the 

 position of the mouth. It either closes or remains permanently 

 open at this point. In some cases the blastopore remains per- 

 manently open and becomes the anus. The best authenticated 

 instance of this is Paludina vivipara, as was first shewn by 

 Lankester (No. 263). 



In some instances the blastopore assumes before closing a 

 very narrow slit-like form, and would seem to extend along the 

 future ventral region of the body from the mouth to the anus. 

 This appears, according to Lankester (No. 262), to be the con- 

 dition in Lymna^us, but while Lankester believes that the closure 

 proceeds from the oral towards the anal extremity, other inves- 

 tigators hold that it does so in the reverse direction. Fol (No. 

 249) has also described a similar type of blastopore. In an un- 

 determined marine Gasteropod, with an embolic gastrula, observed 

 by myself at Valparaiso, the blastopore had the same elongated 



1 The reader is referred for the segmentation to pp. 98 102, and to the special 

 description of separate types. 



