1902.] CONKLIN — EMBRYOLOGY OF A BRACHIOPOD. 55 



and at four places (two median and two lateral) on the dorsal side 

 of the mantle the epithelium is invaginated to form the setae sacs 

 (Figs. 34, 36, 56-64, SS). No traces of setae are preserved in the 

 material which I have examined, but there can be no doubt from 

 Morse's ('73) account that these invaginations give rise to the 

 lateral and median bundles of temporary setae. 



6. Sense Plates and Gafiglia. — A little toward the dorsal side of 

 the apex of the cephalic lobe is a depression in the ectoderm, and 

 in this region the cells are deeply pigmented, especially at their 

 free borders. This is the apical sense plate {Scheitelplatte) (Figs. 

 26, 28, 29, 31, 32, 34, 35, 36, 56, 57, 58, CG), and in life bears 

 a tuft of long cilia (see Morse, '73), though no trace of these is left 

 in the material which I have examined. At the base of the cells of 

 this sense plate ganglion cells are cut off from the epithelial cells, 

 but continue to lie in the ectoderm (Figs. 28, 36, 56, 57). These 

 ganglion cells are small and I have been unable to observe their 

 further development, but there seems no reason to doubt that they 

 represent the cerebral ganglion. 



A similar sense plate and ganglion is formed on the midventral 

 line immediately posterior to the place where the blastopore rem- 

 nant closed and in the region where the blastopore lips fused along 

 the mid-line (Figs. 29, 31, 35, 58, 61, 62, 6'6^). This is the ventral 

 sense plate, and the cells of this plate are pigmented as are those 

 of the apical plate ; I think it probable that they bear a tuft of long 

 cilia in life, although no one has observed this feature as yet. As 

 in the case of the apical plate, ganglion cells are cut off from the 

 basal ends of the epithelium of the ventral plate, and here again 

 ihere seems every reason to believe that these ganglion cells become 

 the subcesophageal ganglion. The oesophagus has not formed in 

 the oldest larva which I have been able to study, but a slight invag- 

 ination of the ectoderm immediately anterior to the ventral sense 

 plate probably represents the earliest step in the formation of the 

 oesophagus (Fig. 58, C?). Heretofore no observations have been made 

 on the early development of the nervous system. Neither Morse 

 nor Kowalevsky observed any stages in the formation of the ner- 

 vous system. Shipley has observed in the head region of Cistella 

 a small clump of cells without granules, which he suggests may be a 

 nerve ganglion ; his Fig. 35 shows however that it lies entirely 

 within the mesoderm, and it cannot therefore be a ganglion. 

 Brooks has described in detail the nervous system of the larva of 



