1902.] 



NATURAL SCIENX'ES OF PHILADELPHIA. 



109 



Explanation of Lettering. 



a.h.l., anterior horizontal lamella. 



aort., aorta. 



art., pseudobranchial artery. 



b.g., basilar (salivary) gland. 



b.m., basilar muscle.' 



hr., point of separation of ciliated 

 and of cover cells. 



b.w., body wall. 



C.6., cartilaginous bars. 



c.d.r., ciliated dorsal ridge. 



ch., notochord. 



c.t., connective tissue envelope. 



d., duct of basilar gland. 



d.c.g., dorsal ciliated groove. 



d.l., cover cells (Decklamelle). 



e., eye. 



f.l., ciliated cells. 



g.l., gland cells (Drusenlamelle). 



g.p , gill pouch. 



gr., primary group of gland cells. 



gr'., secondary group of gland 

 cells. 



i.v., lateral invagination. 



I., loop of the ciliated groove. 



I e.g., lateral cell-group. 



l.c.h., lateral ciliated chambers. 



m., membrane over o. 



m.c.g., 711. eg'., etc., median cell- 

 groups. 



m.c.7i., median ciliated chamber. 



I mes., mesoblast. 



i m.L, median lamella. 



: m.l'., extreme posterior end of 



' median lamella. 



I 71 fl., nuclei of ciliated cells. 



n.gl., nuclei of gland cells. 



0., slit-like opening of cell-groups. 



ces., oesophagus. 



p., side pouch of ventral ciliated 

 I groove. 

 i phar., pharynx. 



p.h.l., posterior horizontal la- 

 mella. 



p.s., protuberance of secreted mat- 

 j ter. 

 ' s., secretion. 



sp.c, spinal cord. 



sto., stomodaeun. 



thy., thyroid gland. 



t.m., tongue muscle. 



U.I., upper lip. 



v., velum. 



v.a.l, 2, etc., 1st, 2d, etc., visceral 

 arches. 



v.c , visceral clefts. 



v.c.g., ventral ciliated groove. 



x-y., line through which the sep- 

 aration of the side pouch from 

 the ventral ciliated groove takes 

 place. 



Explanation of Plates IV, Y, VI, VII. 



All sections draicn with a Zeiss Camera Lucida. 



Fig. la. — Larva of P. pla7ieri, 11 days after fertilization, to show the 

 external appearance of the larva at the time of the first trace of the 

 thyroid. (Mag. 65 diam.) 



Fig. 16. — Transverse section of 12-day larva of P. pla7iei-i, through the 

 anterior end of the thyroid, just anterior to the opening into the pharynx. 

 Shows the gland as a circular cavity lined with columnar cells, lying 

 under the digestive cavity. (Mag. 325 diam.) 



Fig. Ic. — Transverse section posterior to fig. \b, through the anterior 

 part of the opening of the thyroid into the pharynx. (Mag. 325 diam.) 



Fig. 2a. — Transverse section through the extreme anterior end of the 

 thyroid of a 14-day F. platieri. (Mag. 325 diam.) 



Fig. 26. — Transverse section posterior to fig. 2ar., but anterior to the 

 opening of the gland into the pharynx. Shows beginning of lateral 

 invagination. (Mag. 325 diam.) 



Fig. 2c. — Transverse section posterior to fig. 26, through the anterior 

 part of the opening of the gland into the pharynx. (Mug. 325 diam.) 



Fig. 2d. — Transverse section posterior to, fig. 2c, through the posterior 

 part of the opening of the gland into the phar^'nx. (Mag. 325 diam.) 



Fig. 2e. — Transverse section through the gland, posterior to the open- 

 ing into the pharynx. (Mag. 325 diam.) 



