OF MASSACHUSETTS, i'7 



the fact thai the second spawning of the scallop occurs during 1 its old 

 a _;<. and thai the majority of this species do not reach a second season, 

 the eggs of two-year-old scallops may be fertilized and pass through the 

 normal cleavage stages. Although there were some indications that 

 the subsequent development under artificial conditions is not as satis- 

 factory as that of the younger scallops, there is no proof that their 

 development under natural conditions is anything but normal, or that 

 they cannot produce hardy offspring. Naturally, as the two-year-old 

 scallops are few in number their offspring are not numerous. Under 

 artificial conditions as compared with the younger scallops they do 

 not seem to produce spawn so readily and fewer larvae in proportion 

 arc raised to the early swimming stage. These observations cannot 

 be considered as conclusive, as the special Powder Hole set of 1907, 

 already referred to, during 1909 furnished as healthy spawn as the 

 1908 set. 



EMBRYONIC DEVELOPMENT. 



The early life of Pecten irradians can be separated arbitrarily into 

 two main divisions, (1) the embryonic or sub-veliger life, which com- 

 prises the development of the animal until it acquires a shell; and (2) 

 the post-embryonic life of the young scallop before it attains adult 

 characteristics. 



The post-embryonic life is further subdivided into the (a) early 

 veliger stage, when the animal is a free swimming larva with a straight 

 hinge line (Fig. 17); (&) late veliger or prodissoconch stage, distin- 

 guished by the curved hinge and development of gills and foot 

 (Fig. 18) ; (c) the dissoconch stage, where notable changes occur as a 

 result of the "setting," that is, adjustment by spun byssus threads; 

 (d) plicated stage, where the ridges and the furrows characteristic of 

 the adult shell appear. 



The embryonic development of Pecten irradians is in many respects 

 so similar to that of its large relative, Pecten tenuicostatus Mighels, the 

 giant scallop, so ably described by Dr. Gilman A. Drew (1) that it is 

 difficult to present a complete account without a repetition of many 

 interesting facts. For this reason special emphasis has been placed on 

 the points of difference between the two species, and only general con- 

 sideration given those of common interest. In reporting upon this 

 phase of the life history of Pecten irradians, it is perhaps worthy of 

 mention that the results here embodied, imperfect as they are, have 

 been obtained from hundreds of scallops under different conditions and 

 from four years of successive observations. 



The Development of the Egg. 



The development of the egg after fertilization is by the usual process 

 of cell division, whereby the single ovum is transformed into a living 

 mass of tiny cells. Like most lamellibranchs, in which fertilization 



