No. I.] THE EMBRYOLOGY OF THE UNIONIDAE. 7 



fertilized ova of Unio can be obtained from about the middle 

 of June to the middle of July in the above locality; those of 

 Anodonta towards the end of July and early in August. 



The glochidia of Unio escape in August and September. 

 Those of Anodonta are carried by the mother through the 

 winter and extruded finally in the spring. Most remark- 

 ably slow is the process of cleavage in these eggs. The 

 passage from the unsegmented egg to the 45-cell stage takes 

 from five to seven days. I have not been able to get exact 

 data in regard to the time of development, inasmuch as 

 it is impossible to keep the same eggs under observation for 

 more than a very few days. They then die, despite the utmost 

 care. I have seen enough, however, to show the extreme 

 slowness with which the eggs develop. Inasmuch as these 

 ova contain but little yolk, we can only refer this slowness in 

 development to constitutional causes. I know of no observa- 

 tions on the rate of development of the ova of Cyclas. On 

 the other hand, the ova of marine lamellibranchs develop with 

 amazing rapidity. During warm weather the young of the 

 oyster will swim in less than twenty-four hours after fertili- 

 zation. 



(b) Methods. 



The vitelline membrane is separated from the ovum by a 

 wide space (Figs. 2 and 3, PI. I) filled with an albuminous 

 (or mucous) fluid, which coagulates on the addition of the 

 usual killing reagents, making it impossible to obtain clear 

 views of embryos mounted whole. During development the 

 constitution of this fluid alters somewhat, so that this difficulty 

 ceases to exist for stages later than Fig. 79 (PI. V). I was, 

 however, fortunate enough to find a method which gave me, at 

 times, the most perfectly clear views of segmentation stages 

 that could be desired ; at other times it was not so successful. 

 The embryos were exposed to the action of Perenyi's fluid for 

 from ten to twenty minutes ; they were then washed and 

 preserved in seventy per cent alcohol. The material from 

 which the best results were obtained remained in the alcohol 

 for three or four months. The eggs were then mounted, 



