entirely empty, and others at all the intermediate stages, and 
I therefore feel sure that my examinations were made upon 
spawning oysters. 
P While this evidence is for only one season and one bed, I 
think that until it is shown to be exceptional, we must ot 
clude that there is an important difference in the breeding ha F 
its of American and European oysters, and that the “ers 
the American oyster are fertilized outside the body of te 
parent; that during the period which the European oyste? 
passes inside the mantle-cavity of the parent, the young Ameri 
can oyster swims at large in the open ocean. 
he more important points in the development of the oyster 
are : j reed: 
1. The oyster is practically unisexual, since at the b 
ing season each individual contains either eggs or spermatozoa 
exclusively. ‘a 
2. Segmentation takes place very rapidly and follows me 
stantially the course described for other Lamellibranchs ?Y 
Lovén and Flemming. Ae 
3. Segmentation is completed in about two hours, and aa 
rise to a gastrula, with ectoderm, endoderm, digestive CAV’ y 
and blastopore, and a circlet of cilia or velum. At this a 
of development the embryos crowd to the surface of eich ie 
rm a dense layer less than a quarter of an inch thi ae 
4. The blastopore closes up; the endoderm separates 
from the ectoderm, and the two valves of the shell are re 
separate from each other, at the edges of the furrow form 
the closure of the blastopore. “1oted, and 
5. The digestive cavity enlarges, and becomes ciliated, at 3 
the mouth pushes in as an invagination of the ectoderm i 4 
. directly opposite that which the blastopore had occup 
ea 
a 
