30 LILLIE. [Vol. X, 



here with the, stage of Figs. 64, 66, 67, and 72 (PI. V), 

 where the gland is represented by a plate of large cells 

 occupying the whole dorsal ^ region. These cells invaginate, 

 and so give rise to the shell-gland, the long axis of which is 

 transverse to the long axis of the embryo (PL V, Figs. 69 

 and 70); it is of an enormous size as compared with other 

 molluscan embryos. This is, of course, a special provision for 

 the needs of the glochidium, which possesses an enormous 

 shell in proportion to its bulk. So large is this gland that its 

 invagination makes an appreciable difference in the diameter 

 of the embryo, as may be seen by a comparison of Figs. 66 

 and 69, both of which are camera drawings with the same 

 lenses. It might be said that Fig. 69 was drawn from a 

 smaller embryo ; but, as a matter of fact, embryos of this 

 stage are always smaller than in the stage just before the 

 invagination of the shell-gland. A large shell-gland seems 

 to be characteristic of lamellibranch embryos. Reference to 

 figures and comparison with the embryos of other Mollusca 

 will illustrate my point. 



It is not my intention to dwell on the view which interpreted 

 this gland as the archenteron. That is not, I suppose, any 

 longer held by any one. But it is rather remarkable that two 

 observers, Rabl and Schierholz, should have seen the gland 

 migrate bodily to the region in front of the ventral plate, I 

 can only suggest that the observations were made on partially 

 macerated embryos, in which I have myself seen appearances 

 which might deceive in some such way. 



(f) Sufmnary. 



The first cleavage is unequal; the second divides the smaller 

 cell equally and the larger unequally. The four-cell stage is 

 composed of three subequal and smaller cells and one large 

 cell, which lies at the posterior end. One of the smaller cells 

 is anterior and the other two right and left, respectively. The 

 ectoderm is separated from these four cells in a series of three 



^ The term dorsal here refers to the adult, and not the embryonic axis ; the 

 question of axial relations is entered into further on. 



