1879.] 327 [Brooks. 
ectoderm, is formed and entirely covers the four macromeres except 
at a point, the blastopore, which is at the nutritive pole of the egg, 
and directly opposite the polar globules. 
5. The four macromeres now become fused together, forming an 
oval granular mass of protoplasm, part of which separates in such 
a way as to forma layer of cells, the endoderm, arranged around a 
primitive digestive cavity, which opens externally at the nutritive 
pole of the egg, through the blastopore. 
6. The remainder of the granular mass divides up into a number 
of large cells which are situated in the body-cavity between the ecto- 
derm and endoderm. 
7. These large cells are not to be regarded as a food yolk, for they 
grow with the growth of the embryo, instead of decreasing in size, 
and they soon become united to the surface of the digestive tract by 
a layer of mesoderm. 
8. The neck which unites the digestive tract to the blastopore 
now lengthens, its cavity becomes obliterated, and the ectoderm cells 
about its outer end become converted into the shell area, upon which 
the circular symmetrical embryonic shell soon makes its appearance. 
9. The mouth originates as an independent invagination of the 
ectoderm, which makes its appearance on the ventral surface of the 
body, about 90° from the blastopore, but which is soon pushed, by 
the growth of the foot, to a point almost directly opposite the primi- 
tive opening. 
10. It pushes in towards the digestive tract but does not unite with 
it until after the blastopore has closed and become covered by the 
shell. 
11. The stomach cavity appears to be the same as the primitive 
digestive cavity. 
12. After the ‘‘ rectal plug ” has been formed by the lengthening 
and closure of the “invagination neck,’’ its outer end moves from its 
primitive position in the centre of the shell area to a point on the 
ventral surface between the foot and the shell. Here it unites with 
the ectoderm to form the definitive anus, and an axial cavity which 
now makes its appearance converts the rectal plug into the intestine. 
13. ‘The structure and history of the shell area and velum are sub- 
stantially as they have been described by Ray Lankester, and I have 
nothing to add to his account of the later stages. 
14. During segmentation the alternation of periods of segmenting 
activity with periods of rest is very conspicuous. 
