THE DEVELOPMENT OF ALOYONIUM DIGITATDM. 
67 
gloea of the adult polyp. This being so, the endoderm must 
be regarded as its source of origin (see p. 72 seq.). 
The Planula. 
On the fourth day the pre-planula merges into a spherical 
swimming planula of wavy outline (Text-fig. 4). Cilia are 
developed by the ectoderm cells, and a definite anterior pole 
is marked by being held foremost while swimming. The 
coelenteron is still partly filled by groups and strings of 
loose endoderm cells, empty cells, protoplasmic cell contents 
and yolk. The definite endoderm consists of a row of 
columnar cells full of yolk vesicles, and roughly pyriform, 
i. e. rather wider at the free nuclear end than at the base. 
To the inner edge of the permanent endoderm one or more 
rows of rounded cells still adhere as in Text-fig. 36. The 
yolk has all been absorbed in the columnar ectoderm, the 
cell contents being now granular, while the separating 
membrane is much more distinct. These ectoderm cells have 
slightly expanded and flattened bases where they rest on the 
membrane, and are about as tall as the permanent endoderm 
cells, and half as wide. Rapid proliferation will again 
temporarily obscure the columnar character of this layer so 
that it seems to consist of several rows of round cells, but, as 
in the previous and subsequent stages, they soon return to 
their columnar state. In a few hours the round planula has 
become oval, but at first its outline remains sinuous. The 
microscopic structure is little altered, save that the degen- 
erating endoderm cells have been absorbed in increasing 
numbers, while the permanent row of endoderm cells stand 
out clearly (PI. 3, fig. 17, c., End.). By the fifth day the 
outline of the planula is smooth, and soon the anterior pole 
broadens, so that the larva assumes its characteristic pear- 
sliape (Text-fig. 6). It rapidly lengthens, while the permanent 
row of endoderm cells still shows several rows of rounded 
or polygonal cells clinging to it in places, and often forming- 
club-shaped projections into the coelenteron (PL 3, fig. 1, 
.and PI. 3, fig. 17, C. P.j. Increase in the number of endoderm 
