58 KINGS LEY. [Vol. VII. 



Metschnikoff, but of course without actual phyletic connection 

 with it. 



Our knowledge of mesoderm development in the Arthropods 

 is far from complete, and at present it is not possible to point 

 out the peculiarities which characterize the different groups. 

 My account of mesoderm formation, as it occurs in Limulus, 

 agrees well in its major features with the account of Patten 

 (■90), except that he describes at the posterior end of the em- 

 bryo a "slit-like" primitive streak, and he further regards the 

 proliferated cells as both mesoderm and entoderm (p. 373). 

 The lateral connection of mesoderm and ectoderm he compares 

 with the Keimwall of the Vertebrates — a point upon which I 

 would rather admit analogy than actual homology. 



The accounts of mesoderm formation in Scorpio differ. 

 Laurie ('90) describes the inpushing of a mes-entoderm from all 

 parts of the upper (outer) surface of which the mesoderm is 

 afterward proliferated. Patten ('90), on the other hand, de- 

 scribes a median posterior thickening from which cells grow for- 

 ward and laterally, the median portion forming the sexual organs 

 and botryoidal cord ; the lateral, the mesoderm and entoderm. 



In the Decapodous and Isopodous Crustacea the mesoderm 

 would appear to grow forward as two bands from the anterior 

 margin and sides of the blastopore. In some Cladocera and 

 Copepods (Grobben, '79 and '8I) somewhat similar conditions 

 may be traced, except that the primitive mesoderm cells are 

 behind the point of entodermal invagination. In Cyclops, on 

 the other hand (Urbanowicz, '84), mesenchyme is described as 

 budding from the blastoderm cells, and Ulianin ('81) describes 

 the same in Orchestia. 



In the Arachnids our knowledge of mesoderm formation is 

 extremely scanty. All agree, so far as the Araneida are con- 

 cerned, that the primitive cumulus and posterior cloud are con- 

 cerned in the process, and some show that at first the mesoderm 

 forms a continuous band across the embryo. A comparison 

 of figures {e.g. Locy, 86, Fig. 49) of Arachnid embryos with 

 my own of Limulus will, I think, show similarities which cannot 

 be paralleled by similar resemblances between Limulus and the 

 Crustacea. 



In the differentiation of the germ the resemblances of Limu- 

 lus to the Arachnids are striking. So far as I know primitive 



