THE AMERICAN LOBSTER. 217 



This shows that parts of the embryo ou the middle line, including the ocellus, have 

 disappeared, and that the large median pigment spot is beyond question the last trace 

 of the fused compound eyes. 



In fig. 199, which illustrates the third type described by Eyder, the cephalo- 

 thoraces are fused laterally and anteriorly; there is no median eye, but a pair of 

 compound eyes, the right of which is the right eye of the right embryo and the left 

 the left eye of the left. The stomachs are fused on the middle line, but there is no 

 apparent union of the antenna? or mouth parts. Behind the stomach the parts are 

 all double. 



It is evident that the causes which have produced the monsters with a single 

 pair of compound eyes (fig. 199) are of exactly the same nature as those which have 

 produced the monster with a single median pigment spot. In the latter case their 

 action has been more prolonged. It is also evident that these abnormal larva? have 

 been derived from embryos like those which I have described. There is, however, 

 this noticeable distinction : In the abnormal embryos the posterior ends of the bodies 

 are apposed and united, while in the larva? the anterior ends are united, the posterior 

 parts being widely divergent. It seems to me probable that the apposition and union 

 of the tail ends of the embryo do not last long, and never involve anything more than 

 the cells in the yolk; that two distinct thoracic-abdominal processes and two distinct 

 tail folds are formed, which begin to diverge at an early period. Along with these 

 changes a process of fusion apparently takes place between the anterior parts of the 

 two embryos, which are at first entirely separate and distinct, excepting for the yolk 

 and peripheral cells (ectoderm) which unite them. 



The fusion of parts does not take "place coincidently with the process of gastrula- 

 tion," as Ryder suggested, and does not begin to show practical results until after the 

 egg-nauplius stage. In regard to the way in which the different degrees of fusion 

 have been brought about, Eyder applies the rule adopted by Eauber for the interpre- 

 tation of fish embryos — that the degree and manner of fusion is "determined by the 

 width of the angle at which the embryonic axes were primarily inclined to each other." 

 This principle probably applies to the double monsters produced from the lobster's 

 egg, but the process of fusion seems to me to be something entirely distinct from the 

 concrescence seen in the parts of the normal embryo. We have to do here with the 

 fusion of two embryos which are practically distinct from the first. 



NOTE ON THE DEVELOPMENT OF CAMBARUS. 



I received through the kindness of Professor J. E. Eeighard, in the fall of 1893, a 

 large number of crayfish of the species Cambarus immunis. They were taken from a 

 pond in Ann Arbor, Michigan, on the 16th of November, when ice was just beginning 

 to form. Many of the females.were already " in berry," and most of the eggs were in a 

 very early stage of development, some, without segmentation of the yolk, having been 

 recently laid. Several were in the egg-nauplius stage, and the oldest which I ex- 

 amined corresponded to Stage H of Eeichenbach (163), having all the thoracic append- 

 ages, and the folded abdomen had grown forward so that it nearly touched the labrum. 



The eggs carried by each female are relatively very large (1.5 mm. in diameter) 

 and few in number. They are of a light or sometimes rather dark coffee-brown, and 

 appear to be insecurely fixed to the swimmerets, being liable to drop off or become 

 detached, especially when several animals are kept together. The species is hardy 



