660 



were taken from the ovary of an oyster whose long diameter was less 

 than an inch, and which I suppose to be one year old, and fertilized 

 with the semen of a male of the same size, and these eggs completed 

 their development. 



6) Segmentation takes place nearly in the way figured by Lovén 

 in various Lamellibranchs, and byFlemming in Anodonta, and is 

 completed in about two hours. 



7) The embryo has a distinct endoderm and primative digestive 

 cavity with an external opening, and an oral circlet of cilia, and at 

 the end of about two hours after fertilization the free-swimming em- 

 bryos crowd to the top of the vessel and form a dense layer reaching to 

 about half an inch below the surface of the water. 



8) The lips of the orifice of invagination soon approach and unite, 

 so that the endoderm is separated from the ectoderm and its central 

 cavity can no longer be made out. 



9) The two valves of the shell make their appearance, separa- 

 tely, at the ends of the furrow formed by the closure of the gastrula 

 mouth. 



10) On the opposite side of the body an opening which appears to 

 be the definitive mouth pushes in to the endoderm, becomes ciliated : a 

 large digestive cavity lined with cilia is visible within the endoderm ; 

 this becomes united with the opening, and small food granules are 

 visible inside it. 



11) The shells grow rapidly, the anus makes its appearance close 

 to the mouth, and at the end of two days the animals cease to crowd to 

 the surface of the water, and swimm actively at various levels. 



12) From this point the development is very similar to the forms 

 figured by Lovén. 



I was not able to keep them alive in a small quantity of water more 

 than seven days, and they were so small that I was not able to find 

 them in a large quantity of water. And as I could not contrive any 

 way of changing the water without loosing them, I was not able to 

 study the later stages of development. 

 Baltimore, Oct. 11th, 1879. 



2. Zur Embryologie der Bowerbankia. 



Vorläufige Mittheilung von W. Repiachoff in Odessa. 



Die ersten Entwickelungsstadien der beiden von mir untersuchten 

 Bowerbankia-Species l ) stimmen mit den entsprechenden Embryonal- 



1) Vgl. Zool. Anzeiger, 1878. No. 10. 



