MOLLUSCA. 227 



form as in Lymnaeus, 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 my- 

 self, 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 1 . 



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 convenient 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 the blastopore, which then gradually make their way dorsal- 

 wards and forwards, and form a complete layer between the epi- 

 blast and hypoblast. The above general mode of formation of 

 the mesoblast may be seen in fig. 107, representing three stages 

 in the development of Paludina. 



In some cases the mesoblast arises from certain of the seg- 

 mentation spheres intermediate in size between the epiblast and 

 hypoblast spheres. This is the case in Nassa mutabilis, where 

 the mesoblast appears when the epiblast only forms a very small 

 cap at the formative pole of the ovum ; and in this case the meso- 

 blast cells accompany the epiblast cells in their growth over the 

 hypoblast (fig. 105). 



In other cases the exact derivation of the mesoblast cells is 

 quite uncertain. The evidence is perhaps in favour of their 

 originating from the hypoblast. It is also uncertain whether the 

 mesoblast is bilaterally symmetrical at the time of its origin. It 

 is stated by Rabl to be so in Lymnaeus 2 . 



In the case of Paludina the mesoblast becomes two layers 



1 Rabl (No. 268) describes a blastopore of this form in Planorbis which closes at 

 the mouth. 



2 Rabl (No. 268) has quite recently given a more detailed account than previous 

 observers of the origin of the mesoblast in Planorbis. He finds that it originates 

 from the posterior one of the four large cells which remain distinct throughout the 

 segmentation. By the division of this cell two ' mesoblasts ' are formed, one on each 

 side of the middle line at the hinder end of the embryo. Each of these again divides 

 into two, an anterior and a posterior. By the division of the mesoblasts there arise 

 two linear rows of mesoblastic cells the mesoblastic bands which are directed 



152 



