342 PROCEEDINGS OF THE ACADEMY OF [-J^Hiy 



deeply than in other portions of the blastoderm. In the case of the 

 nervous system certain of the ganolia are from the first clearly dis- 

 tinguishable, whereas some others in the organ complex posterior to 

 the shell gland (fig. 23) cannot be distinguished until a later stage. 



a. Cerebral Ganglia. — The group of cells which will form the cerebral 

 ganglia appears at a very early stage. It is shown in figs. 19 and 20 on 

 each side of the apical invagination, and closely connected with the 

 group of cells lying beneath the superficial layer, which I have identified, 

 somewhat doubtfully, with the "larval mesoderm" (fig. 21, Lf7i.). The 

 cells which give rise to the cerebral ganglia arise in the region of the 

 blastoderm, on each side of the anterior arm of the cross and in front 

 of the transverse arms, probably from the "rosette" cells, exactly as 

 in Crepidula, and they probably correspond in origin as well as destiny 

 to the cells of the "cephalic neural plate" of N'ereis (cf. Conklin, 

 1897, p. 110). 



With the great growth of the anterior part of the blastoderm, which 

 follows the flattening out of the apical invagination, these cerebral 

 ganglia are carried laterally until they lie near the margin of the 

 blastoderm (figs. 21, 22, Cb.). In figs. 23 and 24 they lie still nearer 

 the margin, and their protoplasm and nuclei are somewhat more con- 

 densed than in the previous stage; the bases of all the organs now 

 stain more deeply and they are all connected together by deeply stain- 

 ing protoplasm (figs. 23, 24). In all these stages the cerebral ganglia 

 lie anterior to the middle of the blastoderm and are the anteriormost 

 organs present. Later they are carried back until they lie in the 

 posterior lip of the blastopore and at opposite ends of a transverse 

 line which lies nearly in the plane of the first cleavage, and along which 

 line most of the organ bases of the embryo are found (text fig. and 

 figs. 27, 28). Still later, by a continuation of the movement already 

 described, the cerebral ganglia are carried posterior to the first cleavage 

 plane and to the other organs of the posterior lip, and they then ap- 

 proach each other on the ventral side of the embryo, and come to 

 lie on each side of the blastopore (figs. 29. 30). This movement of the 

 cerebral ganglia may be l^etter described by pointing out their position 

 with reference to the shell gland, v.diich may be considered as a_fixed 

 point. In earlier stages the shell gl'^nd forms the apex of an isosceles 

 triangle, the other angles being forined by the cerebral ganglia; then 

 by the separation of the ganglia the triangle becomes equilateral, and 

 still later the angle at the apex increases imtil the ganglia and the shell 

 gland lie in one straight line: then by a continuance of the movement 

 of the ganglia a triangle is formed on the. opposite (ventral) side of 



