No. I.] THE EMBRYOLOGY OF THE UNIONIDAE. 89 



REFERENCE LETTERS. 



a.m. Adductor muscle. 



ap. Apical pole. 



ap. n. Apical nucleus of thread-gland. 



bp. Blastopore. 



e.g. Cerebral ganglia. 



ent. Entoderm. 



f. Foot rudiment. 



h.v. Cells of head vesicle. 



k.a. Kidney anlage. 



/./. Lateral pits. 



mes. tel. Teloblasts of mesoderm. 



mes. Mesoderm. 



mk. Micropyle. 



op. Oral plate. 



p.t.g. Protoblast of thread-gland. 



s. Shell. 



s.g. Shell-gland. 



s.h. Sensory hairs. 



t.g. Thread-gland. 



v.p. Ventral plate. 



A Left macromere. 



B Anterior macromere. 



C Right macromere. 



D Posterior macromere. 



01, ^i, c^, </i, a'-i, etc., First generation of ectomeres. 



a2, ^2, f2, d^, a 2.1, etc.. Second generation of ectomeres. 



a^, bz, a, dz, etc., Third generation of ectomeres. 



X = d2 First somatoblast. 



M= d4 Second somatoblast. 



y = a 2.2 Larval mesoblast. 



DESCRIPTION OF PLATES. 



All figures were drawn with a camera lucida under a magnification of 275 

 *ameters, except where otherwise stated. With two exceptions (Figs. 3 and 8) 

 no attempt has been made to show the actual appearance of the segmenting 

 ovum. A light, uniform shading has been adopted throughout for the sake of 

 clearness. This of course does not hold for the other figures of Plates V and VI. 



