No. I.] THE EMBRYOLOGY OF CREPIDULA. 109 



3. Organs formed from the First Quartette. 



The following organs, which I have studied with more or 

 lees care, are formed from the first quartette : the umbrella, or 

 " head vesicle," an apical plate of ciliated cells, the posterior 

 cell plate, a portion of the velum, the supraoesophageal ganglia, 

 an apical sense organ, a commissure connecting the ganglia 

 with each other and with the apical organ, the cerebro-pedal 

 connectives, and the eyes. 



(a) The Head Vesicle reaches its maximum development 

 before the veligers escape from the egg capsules ; in fact it 

 decreases in size as the velum increases, the walls of the 

 vesicle being drawn out into the velar lobes. In its fully 

 formed condition it is a large bladder-like structure, filled 

 with a transparent fluid. The walls of the vesicle are but one 

 cell thick in early stages, though in later stages a few scatter- 

 ing cells, probably mesoderm, are found on its inner surface. 

 As the head vesicle is formed the apical cells are pushed far- 

 ther and farther forward, and the vesicle is composed almost 

 entirely of the large ciliated cells which lie posterior to the 

 transverse arms, viz., the posterior turrets and the basal and 

 middle cells of the posterior arm. These cells form a more 

 or less definite structure, lying posterior to the apex, which I 

 have designated the posterior cell plate (P-C, Figs. 74-82). 



(b) The Apical Sense Organ. — The four apical cells can be 

 still recognized in Fig. 79. In this figure, and also Figs. "jZ and 

 96, it can be seen that these cells are somewhat indented over 

 their outer surface, and have proliferated a few cells inward 

 into the cavity of the head vesicle. This mass of cells, to- 

 gether with the four apical cells from which it arose, forms an 

 organ which soon comes into relation with the supraoesopha- 

 geal ganglia by means of a strand of cells which grows out from 

 those ganglia. This structure is, I believe, an apical sense 

 organ, and it is located exactly at the point at which the polar 



to the size of the part to which it gives rise, as is shown by the case of the trocho- 

 blasts cited above, but it is frequently true that the initial size of a blastomere is 

 directly related to the size of the part to which it gives rise and to the time of its 

 fo7-matio7i. 



