No. 2.] DEVELOPMENT OF MANICINA AREOLATA. 203 
see there is a well-defined lamella, which is formed zz sztu. In 
Cunocantha (7) I have pointed out that when the velum is 
developing, the ectoderm cells range themselves so that their 
bases will secrete a continuous membrane. 
IV. FoRMATION OF THE First Parr oF MESENTERIES AND 
FILAMENTS. 
I was led to pay especial attention to the filaments by E. B. 
Wilson’s interesting discovery that the dorsal pair of filaments 
in the Alcyonarian polyps are ectodermal lobes (8). Relying on 
the histological similarity between this pair of filaments and the 
flimmerstretfen of an actinian filament, Wilson suggested that 
perhaps in the latter the /@mmerstreifen were ectodermal, and 
only the median xesseldrusenstretf was endodermal. I find, how- 
ever, that the filaments of Maniciva are as to origin like the 
dorsal pair in the Alcyonaria. They are simple ectoderm lobes 
which grow down from the cesophagus. 
The cesophageal invagination in the earliest stages is symmet- 
rically placed, Figs. 4 and 5. It very soon, however, begins to 
travel towards one side of the larva. This is shown in the 
transverse section, Fig. 7, and still better in the longitudinal 
section, Fig. 6 (the endoderm in these two larve is in different 
stages of development). The lateral motion of the cesophagus 
has compressed the endoderm on its left into a compact mass, 
which completely fills the space between the cesophagus and 
body ectoderm. On the right hand the endoderm has been 
stretched, until it forms a single layer of cells. The narrow 
lumen of the cesophagus is bilateral (it is doubtful whether this 
is true in all larvee at this stage), and its dilated extremity has 
no longer the symmetrical bulb-like shape of Fig. 5. In Fig. 8 
the lateral movement has gone a step farther, and meanwhile 
the cesophagus has opened centrally. The movement of the 
cesophagus is continued until in one meridian there is nothing 
left between the cesophageal and superficial ectoderm but the 
supporting lamella, Figs. 11 and 12. (In Fig. 11 the lamella 
has not yet appeared, but both stomadoeal and body ectoderm 
have smooth, limiting surfaces.) The intermediate stages show 
that the lateral movement of the cesophagus travels from above 
downwards, and that the endoderm has consequently been pushed 
