570 LILLIE. [Vol. VIII. 



teloblasts from the cells of the shell-gland, remains as the 

 status of our present knowledge on the subject. I shall show 

 that the origin of the mesoderm teloblasts in Unio is exactly 

 the same as in Nereis, according to Wilson's ^ description. In 

 addition to this yet another source of origin exists in Unio, viz., 

 from a cell on the left side of the young embryo of forty-six 

 cells (Fig. 4, F). I shall call the mesoblast derived from this 

 source the larval mesoblast, inasmuch as it gives rise to 

 structures which are purely larval. 



Segmentation. 



The first plane of division is inclined at an angle of 45° 

 to the future sagittal and transverse axes of the larva. It 

 runs from the animal to the vegetative pole, and divides the 

 Qg^ into two unequal parts, of which the smaller A B \^ anterior 

 to the larger CD. Both cells contain entoderm as well as 

 ectoderm, and, therefore, Rabl's designation of animal and vege- 

 tative does not correspond with fact. The second plane of 

 division runs likewise from the animal to the vegetative pole, 

 and is practically at right angles to the first. It does not divide 

 both cells at the same time. C D \^ the first to divide ; the two 

 resulting cells are of unequal size, and the smaller C lies on 

 the right side of the future embryo, while D occupies the median 

 posterior region (Fig. i). A B divides shortly afterwards and into 

 parts approximately equal, one of which A {slightly the larger) 

 lies on the left side and the other B occupies the median an- 

 terior region. The median longitudinal axis of the future em- 

 bryo runs through the centres of B and D (Fig. i m. I. p.). 



So far my description of the segmentation agrees completely 

 with Rabl's and Flemming's^; but my interpretation of the 

 value of these first four blastomeres, and their axial relations, 

 is entirely different. Rabl held that the large posterior cell, 

 Z> (i of Rabl), was chiefly entodermic, while, as a matter of 

 fact, it contains only a very minute portion of entoderm ; that 



1 Wilson, Cell-Lineage of Nereis. Journal of Morphology. Vol. VI, No. 3. 



2 Flemming, Studien in der Entw. der Najaden. Sitzungber. der Wiener 

 Akad. 1875. 



