50 KINGSLEY. [Vol. VII. 



in all. Figs. 27 and 28 show two modifications which I have 

 witnessed, the latter in two instances. In the first and more 

 normal of these figures the cephalic region is small, and behind 

 it come six somites, each with the outline of a pair of appen- 

 dages. The sixth appendage is faint, and the segment which 

 bears it is not yet differentiated from the abdominal region. 

 The same state of affairs is shown in the slightly later stage 

 represented in Fig. 29, made from an embryo peeled from the 

 egg, and which also shows several other points to be described 

 later. In this the mesodermic somites are obscured, while the 

 abdominal region is more elongate. On the other hand, Fig. 28, 

 also made from a transparent specimen, shows but jive pairs 

 of thoracic feet, while in other respects the embryo is much 

 further advanced, as is shown by the existence of appendages 

 VII and VIII (operculum and first gill-bearing appendage) in 

 the abdominal region. 



This variation in the time and order of the appearance of the 

 appendages probably explains the difference between Dohrn 

 ('71) and Packard, the former stating that appendage I appears 

 later than the others. This is certainly true in some cases, but 

 out of several hundred eggs examined at about the time of the 

 appearance of the feet, I have seen but two instances, and in 

 my former papers ('85 and '90) I took Packard's position, as at 

 the times when those papers were written I had not seen a 

 specimen without appendage I. 



Professor H. L. Osborn ('85, p. 2) gives the following account 

 of the appearance of the limbs in Limulus : " On July 28th, 1 1.30 

 A.M. [the eggs were fertilized July 23], a deep semicircular de- 

 pression showed itself. On the 29th, in the space between the 

 two lips of the depression of the day before, a pair of buds ap- 

 peared — the beginnings of the anterior pairs of limbs. On the 

 following day two more pairs are added, and in front of the first 

 pair and behind the front lip of the fold a most important struc- 

 ture is for the first time seen : it is a slit elongated antero- 

 posteriorly, — the definitive mouth opening. It is distinctly in 

 front of the first pair of limbs. It is to be noted that the anal 

 opening has not yet shown itself, according to my observations. 

 The stomodaeum and the three somites are now included in 

 an area which is plainly marked off from the rest of the egg 

 and surrounded by an oval elevation. On the following day. 



