ONTOGENY AND PHYLOGENY. SjOOUg. 7 
but sometimes, though rarely, the order is reversed. On first escaping 
from the parent the blastula seeks the light ; but on the approach of gas- 
trulation, which usually occurs towards evening, it avoids it. The 
gastrula is very quickly formed, and is at first hemispherical ; the blastula 
cavity 'is not completely obliterated, but remains as a considerable space 
between the two layers, which retain at first the same structure as they 
possessed in the blastula. The blastopore is a wide circular opening, the 
c.ells surrounding it of the same character as all the rest. 
(d) Stage of attached Gastrula. — Almost immediately after invagination 
the gastrula attaches itself by the margin of the blastopore. It then 
becomes depressed, and broadens out into a flat cap-like form, it grows 
clearer, pigment disappears from the ectoderm and is more uniformly dis- 
tributed in the endoderm. The ectoderm and endoderm are now histo- 
logically differentiated : the cells of the latter still resemble those of the 
blastula, except that their basal portion is no longer sharply defined 
against the albuminoid jelly, and their collars have grown longer; the 
cells of the ectoderm have undergone great changes, they are no longer 
prismatic but cubical, the distinction between exo- and endo-plasm is 
abolished, the cell is more hyaline and less granular, and its outer face 
shows a cuticular thickening, the nucleus has retreated to the base, the 
collar has disappeared, but the flagellum is retained. 
(e) Commencement of the closure of the blastopore. — The blastopore 
grows narrower, and by the uniform extension of its margin inwards the 
gastral cavity becomes bounded below ; simultaneously the endoderm com- 
mences to undergo a process of folding which finally results in the forma- 
tion of the system of flagellated chambers. Pseudopodia- like processes 
of attachment are now seen extending* from the margin of the larva; 
which has now a flat cake-like form, with rounded edges and a flat 
summit. 
(f) Morphological differentiation of the endoderm.— 'While the blastopore 
still remains large, the endoderm grows into a discontinuous fold, which 
runs parallel with the margin of the gastral sac, with its greatest exten- 
sion towards the aboral pole. Next radial folds appear, which extend 
from the sides iuwards, septa-like. The blastopore is now considerably 
narrower. The radial folds are next transversely segmented into an inner 
and outer portion : thus result two concentric circular rows of diverti- 
culae — the rudiments of the flagellated chambers. A similar arrangement 
of the flagellated chambers at a corresponding stage is described by 
Schulze in Placina. 
( g ) Further changes up to the closure of the blastopore. — The distinc- 
tion between the cells of the flagellated chambers and those lining the 
gastral cavity is now established. All the cells have now lost their pris- 
matic form, and none contain yolk-granules. The cells of the chambers 
are flask- or pear-shaped, with high collars, and a basally situated nucleus ; 
the others, lining the general cavity, are cubic, with a round, centrally 
situated nucleus, without a collar, but still possessing a flagellum, indeed, 
it would seem that flagella are not retracted from the cells generally at 
any stage of the development. The albuminoid jelly of the blastula- 
cavity has increased in bulk, and fills a considerable space at the edge of 
