No. 2.] EMBRYOLOGY OF THE AMERICAN LOBSTER. 249 



dots, midway between the compound eyes, are the two halves of 

 the ocellus, which has been described for several of the higher 

 Crustacea. Its absence in Astacus is of interest. The tip of 

 the thoracico-abdominal fold is being constricted off from the 

 proximal portion to form the telson, and the rudiments of tho- 

 racic appendages have made their appearance. 



Stage P. 



The compound eyes have become pigmented, the ocellus is a 

 single median spot, the sloughed cuticle is still present, and the 

 thoracico-abdominal portion is much elongated. 



The presence of terminal telson spines, of additional appen- 

 dages, and of the clearly outlined posterior edge of the carapace 

 should be noted. The latter appears as a crescent encircling 

 the posterior portion of the embryo. It fades away insensibly 

 at a little distance from the embryonic tract. 



Stage Q. 



The tip of the telson has now reached a point even with the 

 ocellus. Additional appendages are to be seen, which are grow- 

 ing over the flexed abdomen. 



Stage R. 



The embryo has now reached a considerable size. The telson 

 extends anteriorly, in front of the head. The thoracic appen- 

 dages have grown over and partially covered the abdomen. The 

 most characteristic feature, however, is the presence of clearly 

 defined areas which encircle the yolk and partially hide under- 

 lying structures. The embryo has now reached a stage which 

 may be compared with the earliest stage figured by Smith (71). 



Brown University, Providence, R.I., 

 December, 1890. 



