THE LAMELLIBRANCHIA 245 
a 
in the cloacal or suprabranchial chamber, as in Cardiwm and several 
other ineubatory forms, or in the oviduct itself in Ostraea edulis. 
The formative pole of the ovum is opposite to the micropylar 
end. The segmentation is unequal from the first cleavage onwards. 
The macromere formed at the first cleavage is loaded with yolk 
granules and remains single for a long time, but gives rise to the 
three first groups of micromeres, which partly cover it as with a 
cap (Fig. 9, C). Finally, the macromere divides to form the endo- 
derm cells. The gastrula is rarely formed by invagination (Pisidium, 
Ray Lankester), but in nearly all cases by epiboly, or sometimes 
by a process midway between the two, in which there is at first 
an epiboly resulting from the multiplication of the small ectodermic 
cells surrounding the single macromere, and finally an invagination 
after division of the macromere. This process is found in Ostraca, 
Cyclas, and the Unionidae, and in the two last-named the segmenta- 
tion cavity is very large and the enteron small (Fig. 227). The 
blastopore remains open in some cases, e.g. in Ostraca, but closes in 
Cyclas, Pisidium, the Unionidae, Dreissensia, Teredo, etc. But the 
mouth and oesophagus are soon formed by a secondary ectodermic 
invagination at the point of closure of the blastopore. The yolk 
remains in connection with the dorsal surface of the enteron and is 
slowly absorbed. The endoderm gives rise to the stomach and the 
two liver lobes and to the intestine ; the liver lobes often display 
a marked asymmetry, the left lobe being larger than the right in 
Mytilus, Dreissensia, and Yoldia. The anal ectodermie invagination 
placing the intestine in communication with the exterior is gener- 
ally at the extreme posterior end of the embryo, is very short, and 
very late in appearance. The mesoderm originates, at an early 
period, from the most posterior of the four primary endoderm 
cells ; the resulting mesomeres take up a position between the 
ectoderm and endoderm in the form of two symmetrical mesoderm 
bands. 
In its general characters the development of the Lamelli- 
branchia conforms to the type observed in the other classes of 
the Mollusca, but a certain number of special features must be 
noted. (1) The shell-gland makes its appearance at an early 
period in the normal position; that is to say, at the formative 
pole, nearly opposite to the blastopore (Fig. 223, sk). It is single, 
like the shell-gland of all other Molluscs. During its extension it 
gives rise to a saddle-shaped cuticular pellicle, which becomes 
calcified at two symmetrical points, right and left of the middle 
line. These two centres of calcification eventually form the two 
valves of the shell, but, except in the Unionidae, they do not 
develop as fast as the subjacent lobes of the mantle. The two 
valves remain united by the median and dorsal part of the primitively 
single shell, and the ligament is formed at this line of union. 
