No. 6. — Studies in Morphogenesis. — I. On the Development of the 
Cerata in Molis. By C. B. DAvENPOnT.! 
Waite at Mr. Agassiz’s Newport Laboratory last summer, I collected, 
early in August, some spocimens of /Eolis? which oceurred abundantly 
on the ITydroids. The individuals varied greatly in size, and, since Nu- 
dibranch eggs had been abundant on the same Hydroids earlier in the 
season, I regarded most of them as immature. 
I was particularly struck by the regularity with which the cerata, or 
dorsal papillee, were distributed over the surface of the body, and the 
constancy in position of young cerata in relation to the older ones. 
Figures 1 and 2 are drawings of two individuals showing the arrange- 
ment of the cerata. They are placed in transverse rows, which are not 
equidistant, however, but grouped in twos or threes. In each transverse 
row the largest ceras lies dorsalmost, the smaller more ventral, the 
youngest of all being often indicated by only a slight protuberance at 
the base of and ventral to the next older. The regularity of the pro- 
cess of budding of new cerata induced me to make sections to determine 
its details, 
The cerata of Æolis contain, as is well known, processes of the ali- 
mentary tract, — the so called hepatic coa (Plate II. Fig. 12). They 
are covered externally by the ectodermal epithelium. Between ectoderm 
and hepatic cocum are mesodermal cells, which lie (1) in a thin layer at 
the inner surface of the ectoderm, (2) in a thin layer over the cocum, 
and (3) between these two layers, surrounding blood spaces or loosely 
distributed. 
The coecum of each ceras connects at the base with a lateral diverticu- 
lum of the alimentary tract, — gastric diverticulum, — with which also 
the coca of all of the other cerata of the same transverse row unite 
(Plate I. Fig. 3, ga. dv.). There are as many of these gastric diverticula 
as there are transverse rows of cerata (Fig. 4, ga. dv.). 
1 Contributions from the Zoólogical Laboratory of the Museum of Comparative 
Zoology, under the direction of E. L. Mark, No. XXXVII. 
2 Owing to the immaturity of the individuals, I was unable to determine the 
species accurately. Figures 1 and 2, however, show the external form of the 
species (one or two) employed. 
VOL. XXIV. — NO. 6, 
