THE DEVELOPMENT OF LAMELLIBRANCHS. 415 



mesoblastic cells appear to be derived both from the epiblast 

 and the hypoblast. 



Tlie mouth is formed by a depression of the ectoderm at 

 the anterior end of the body, which grows toward and opens 

 into the archenteron. The anus is developed at the opposite 

 end, in the region of the primitive invagination. On the 

 neural face of the embryo the foot grows out, while the mantle 

 appears on the opposite face ; and, in the centre of the man- 

 tle, a transversely oblong depression lined by elongated cells 

 is the " shell gland." In the median line this answers to the 

 ligament, and, at the sides, to the middle region of the future 

 valves of the shell ; but the precise share, if any, which it 

 takes in the formation of these parts does not appear. Pisi- 

 dlum has no velum. 



The development of one of the fresh-water Mussels ( Unto 

 pictorum) has recently been worked out very fully by Rabl. 1 

 The vitellus divides into two unequal masses, of which the 

 larger is termed by Rabl the " vegetative " and the smaller 

 the " animal " cell — somewhat inconvenient names, which may 

 be replaced by " macromere " and " micromere." Each of 

 these becomes subdivided, partly by ordinary fission, partly, 

 as in the case of the macromere, by a process of budding, into 

 blastomeres, of which those which proceed from the macromere 

 long remain larger and more granular than those which pro- 

 ceed from the micromere. The blastomeres arrange them- 

 selves into a hollow sphere — the blastosphere. This is a vesic- 

 ular morula, composed of a single layer of blastomeres, of 

 which those of one hemisphere have proceeded from the micro- 

 mere, and those of the other from the macromere. Two blas- 

 tomeres of the macromeral hemisphere remain much larger 

 than the rest. The macromeral hemisphere next undergoes 

 invagination, and its invaginated part becomes the hypoblast. 

 The two large blastomeres just mentioned, which are disposed 

 symmetrically, one on each side of the median plane at the 

 anterior margin of the area of invagination, become inclosed 

 between the hypoblast and the epiblast, and by their division 

 give rise to the mesoblast. This last, therefore, may be re- 

 garded as an indirect product of the hypoblast. 



The endodermal sac formed by the hypoblast now loses its 

 connection with the region of the embryo of which it is an 

 invagination, and applies itself to the anterior wall of the 

 body, where an involution of the ectoderm, which gives rise 



1 C. Rabl, " Ueber die Entwickelungssreschichte der Malermuschel," Jena, 



1876. 



