I g5 KINGSLE Y. [Vol. V 1 1 L 



Introduction. 



The preceding portion of this paper (this Journal, Vol. VII, 

 p. 33) dealt with the habits, ovigenesis, the origin of the germ 

 layers, and the development of the external form of Livmlus 

 polyphcmus, together with some more general questions con- 

 nected with the matters under discussion. In the present part 

 the development of some of the organs is followed out, and in 

 conclusion I have considered the bearings of the facts upon the 

 systematic affinities of the Xiphosura and upon the classification 

 of the Arthropods. 



I would, before beginning, call attention to the fact that, 

 disregarding the neuromeres, I have numbered the somites and 

 the appendages, beginning with the first appendage. This may 

 account for some apparent discrepancies later, and it also 

 brings my article into harmony with those of other writers. 

 At present we are in a transition condition, and no other 

 course seems advisable. The matter of the neuromeres will 

 be discussed in its proper place. 



The Mesoderm and its Derivatives. 



As previously described the mesoderm arises by cell prolif- 

 eration from a median longitudinal line on what will eventually 

 form the ventral surface of the embryo. For this line of 

 mesodermal formation the name primitive streak, borrowed 

 from vertebrate embryology, seems especially appropriate. 

 From this streak {cf. Figs. 42, 43) the middle layer grows 

 outward on either side between ectoderm and entoderm (yolk), 

 the process of differentiation beginning in front and gradually 

 extending backwards, where the process continues for some- 

 time after it has ceased anteriorly and after the coelomic 

 pouches have formed in the first somites of the body. This 

 fact gives confirmation of the correctness of interpretation of 

 anterior and posterior in the earliest embryos {vide '92, p. 47), 

 a point upon which absolute certainty was impossible, since 

 the features there described only become visible upon the em- 

 ployment of methods which coagulate the albumen and conse- 

 quently kill the egg. 



