MUSEUM OF COMI'AKATIVE ZOOLOGY. 145 



After having fouiul that tlie cerata arose in close connection with pre- 

 ceding ones of the same transverse rows, and therefore ultimately in 

 close connection with the first or. dorsal most ccras of each row, it seemed 

 desirable to determine the origin of the dorsalmost longitudinal series of 

 ccrata, and of the origin of new transverse rows. Sagittal, or slightly in- 

 clined longitudinal, and also frontal sections of young specimens gave the 

 desired information. Figure 16 represents a longitudinal section which 

 is not strictly sagittal, the upper edge of the section plane having been 

 tilted about 30° towards the right so as to pass at the same time through 

 the alimentary tract and the dorsal • series of cerata, — the only series 

 as yet developed on that side. From the drawing it is clear that the 

 alimentary tract sends off diverticula, which pass directly into the first 

 cerata of each transverse row. Behind the most posterior ceras the 

 mesenchyme is thickened over the alimentary tract, which runs to the 

 posterior end of the body. In a section a little removed from this 

 (Fig. 17, II.), one sees the beginning of another papilla behind number 

 III. of the series represented, — the foundation of another transverse 

 row. The ectoderm has already begun to fold upwards, and the alimen- 

 tary tract sends out a prominent pocket. 



Of very great importance is the fact that just behind the Anlage of the 

 ceras the testis is arising, so that the mesenchyme which seems to take 

 the initiative in the formation of the ceras gives rise also to the sexual 

 cells. Moreover, the two cell masses arise close together, and indeed in 

 a definite relation to each other. As is clear from an inspection of 

 Figure 16, the sexual glands — ovary and testis — lie between the trans- 

 verse planes occupied by the cerata, and in each mass the ovary lies in 

 front of the testis, so that successive transverse sections cut from the 

 head backwards pass in oi'der through a transverse row of cerata, through 

 an ovarian mass, and through a testicular mass. This succession is, 

 however, not that in which the Anlagen of the three organs have been 

 established out of the mesenchyme of the tail end; for, as Figure 17 in- 

 dicates, new sexual cells arise before the ceras which lies in front of them 

 begins to appear. 



The oblique (nearly frontal) section, Figure 18, shows the same relation 

 of the sexual glands and the cerata. A reconstruction of the series shows 

 that diverticula arise from the parts of the alimentary tract indicated by 

 the designations cer. I., II., III., and IV. These diverticula correspond 

 in position to transverse rows of cerata. The same i-econstruction shows 

 that between these diverticula ovar}' and testis follow in the way just 

 described. At the extreme tail end the section passes obliquely through 



