_1 "_ 



slit -like opening into the pharynx until only a small circiilar 

 canal is left (no section through this opening is represented) ; 

 the thinninr of what we shall hereafter call, after Dohrn, the 

 "cover"oells ( "Decklamelle") (Fig. 4 b, d.l.), and the thickening 

 and deeper invagination of the "gland" cells ( "Drusenlamelle") 

 (Fig, 4 b, g.l.). TThe part of the gland posterior to what we 

 may call now the "duct" remains about as in the preceding stage 

 (Fig. 4 c). 



The larvae of the next stage were about nine mm. long, 



A j% 



and showed all the outward characteristics of the normal ammo- v 



a. 



coetes. An outline drawing, from life, of the side of the head 

 is shown in Fig, 5 a. Being drawn under the microscope with a 

 camera, the relative size and position of the thyroid are accu- 

 rately shown. The gland is seen to extend from the first to the 

 fifth gill arch, and to be closely wedged in between the pharynx 

 above and the body wall belov/. The upwardly curved ends did not 

 show in the living animal, except as indicated by the upward 

 bending of the longitudinal grooves; neither did the duct leading 

 to the pharynx, so these features were omitted in this illustra- 

 tion. The longitudinal grooves are exaggerated somewhat in the 

 drawing. The gland, in life, had a faintly yellov/ color, while 

 the rest of the larvae, except the blood, was nearly v/hite, 

 with numerous many-branched, black pigment spots resembling very 

 ccmpllcated nerve cells. The living larva of this age is quite 

 transparent, so that the pulsation of the heart and of the velum 



