202 KINGSLEY. [Vol. VIII. 



cavity) migrates to the dorsal surface where it comes to lie at 

 either side of the central circulatory organ. Kishinouye says 

 it disappears before hatching. I do not find that it does so 

 but find it persisting {cf. Fig. 87) after the assumption of the 

 adult characters, for a portion of the length of the body. My 

 belief that it gives rise to the gonads was referred to above. 



Our knowledge of mesoderm formation in the Arachnida is 

 deficient, but as far as it goes it agrees well with that of Limulus. 

 Thus all accounts agree in the following : The mesoderm arises 

 by a proliferation from a median ventral primitive streak, the 

 proliferation continuing later in the posterior region than in 

 front ; next the mesoderm (+ entoderm in Euscorpius) separates 

 from the ectoderm and forms at first a broad sheet across the 

 ventral surface of the embryo ; and, next, this sheet becomes 

 divided into right and left bands connected behind ; and into 

 distinct somites in either half of the body. Later in each half 

 of each somite a coelomic cavity is formed by splitting and these 

 cavities extend temporarily into the corresponding cephalotho- 

 racic appendages. This will apply equally well to Limulus, 

 but farther, differences in details are to be noted. Thus Balfour 

 ('80, p. 174, PI. XX, Fig. 13) describes cells from the yolk 

 migrating into and adding to the mesoderm, while in Euscorpius 

 (Laurie "90) the mes-entoderm is first differentiated from the 

 ectoderm and then later it is divided into mesoderm and ento- 

 derm. From what we now know of Arachnid development I 

 think it safe to regard the view of Balfour that the mesoderm 

 was increased by migrations from the yolk as erroneous, 

 certainly his figure ('80, PI. XX, Fig. 13) does not require such 

 interpretation, while the differences in scorpions is readily un- 

 derstood in connection with the peculiar features of the for- 

 mation of the entoderm already discussed (Part I, p. 53 ff.) 



Both Laurie (Scorpion) and Balfour (Agalaena) describe a 

 prestomial coelom but without stating whether it actually be- 

 longs to the prestomial area or migrates to that region as in 

 Limulus. Kowalevsky and Schulgin, on the other hand ("86) 

 describe in the scorpion a preoral somite with a distinct cavity. 

 Laurie has also studied the genital duct which according to him 

 develops from the coelom of somite VIL Owing, however, 



