MOLLUSC A. 187 



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. According 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 gas- 

 trula becomes formed either by normal invagination as in the case of 

 Paludina (fig. 107), or by epibole as in Na-ssa mutabilis (fig. 105). In 

 both cases the hypoblast becomes completely enclosed by the epiblast. 

 The 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 permanently 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 condition in Lymneeus, 

 but while Lankester believes that the closure proceeds from the oral 

 towards the anal extremity, other investigators hold that it does so 

 in the reverse direction. Fol (No. 249) has also described a similar 

 type of blastopore. In an undetermined marine Gasteropod, with an 

 embolic gastrula, observed by myself at Valparaiso, the blastopore had 

 the same elongated form as in Lymnseus, but the whole of it soon 

 became closed except the oral extremity; but whether this finally 

 closed could not be determined. It is probable that the typical form 

 of the blastopore is the elongated form observed by Lankester and 

 myself, in which an unclosed portion can indifferently remain at 

 either extremity ; and that from this primitive condition the 

 various modifications above described have been derived 2 . 



Before the blastopore closes or becomes converted into the oral 

 or anal aperture, a number of very important embryonic organs 

 make their appearance; but before describing these it will be con- 

 venient to state what is known with reference to the third embryonic 

 layer or mesoblast. 



This layer generally originates in a number of cells at the lips of 



1 The reader is referred for the segmentation to pp. 80 83, and to the special 

 description of separate types. 



- Babl (No. 268) describes a blastopore of this form in Plauorbis which closes at the 

 mouth. 



