I gS KINGS LE Y. [Vol. VIII. 



by Fig. 45, which is a sagittal section through an embryo with 

 seven differentiated somites, the mesoderm extends farther 

 toward the middle line in a segmental than in an interseg- 

 mental area. Had the section passed further from the middle 

 line the mesoderm would have formed a continuous sheet from 

 head to caudal region. 



The ccelom arises (Fig. 43) by a splitting of the lateral 

 halves of the mesoderm, a pair of cavities being thus formed 

 for each somite. These coelomic cavities long remain distinct 

 from each other, and the dissepimental walls persist until a 

 later stage of development. The later history of the coelom is, 

 however, very difficult to follow on account of the subsequent 

 appearance of numerous lacunae in the mesoderm, which, so 

 far as my observations go, have no connection with the prim- 

 itive cavities which I would homologize with the coelom of 

 other forms. The coelomic pouches appear in the six thoracic 

 segments before they do in those of the abdomen (Fig. 46). 

 An especially interesting fact is that there is no preoral 

 coelomic pouch, but the cavities of the first postoral somite 

 send prolongations (Fig. 47) into the region in front of the 

 mouth. 



The walls of the coelom differ in thickness. The somato- 

 plure is usually several cells thick, while the splanchnoplure 

 is only one cell in thickness and rapidly takes the form of a 

 thin layer of pavement epithelium closely applied to the under- 

 lying yolk. A partial exception to this general statement 

 occurs in the abdominal region where (Fig. 49) the somato- 

 plure at first may also be a single cell thick. With the 

 appearance of the limbs, as I pointed out in my earlier paper 

 ('85, p. 532), the coelom extends into these members, but it is 

 soon excluded by the rapid growth of mesoderm in their 

 interior. Of the fates of the coelomic pouches I cannot speak 

 with absolute certainty. The following is what appears to me 

 the probable history. In all of the thoracic segments a portion 

 (if not all) of the coelom is gradually carried, with the advancing 

 mesoderm, from the ventral on to the dorsal surface of the 

 embryo. That a portion is thus carried is certain ; but the 

 rapid formation of lacunar spaces in the somatoplure renders 



