378 Kalph S. Lillie 



lamella, as will be shown beyond. As tlie iiumber of meseuchyme 

 cells behind this partition increases — chiefly by the multiplieation 

 of the large polar nuclei — an irregulär hollow S})ace appears 

 between the cells in tlie neighborhood of the partition, and gradually 

 eularges, apparently in part by the absorption of fluid from the 

 adjoining coelomic cavities. The space so formed represents the 

 early coelomic cavity of the somitc which is in process of formation; 

 it becomes progressively better defined as growth proceeds, while 

 the laycr of mesoblast forming its walls becomes irregularly applied 

 to the surfaee of adjoining structures — anteriorly to the posterior 

 wall of the next somite in front, exterually to the ectoderm of the 

 body-wall, internally to the wall of the intestine and posteriorly to 

 thc densely packed mesoderm of the growing region behind. The 

 partition formed by the apposition of these mesoblast cells to the 

 posterior wall of the somite in front represents the earliest conditio« 

 of the newly forming intersegmental septum, and thus defines the 

 anterior boundary of the newly forming somite. The early septum 

 thus consists esseutially of the oppcsed walls of this incipieut somite 

 and of the somite in front. Its characteristics and relations to thi 

 nephridia will shortly be cousidered in detail. 



The bilaterality of the early mesoderm is not very decidedly 

 pronounced. A ventral mesentery is apparently not formed in the 

 anterior region of the body and the coelomic cavities of the two 

 sides are therefore from the first continuous below the intestine. 

 Dorsally, however (Piate 23, Fig. 13), the right and left coelomic 

 cavities remain distinct, and the adjoining walls (which soou assume 

 a membranous charaeter) become applied to one another above the 

 intestine, forming a bilaminar membrane, whose lamellae are from 

 the conditions of their formation necessarily continuous with the 

 mesoderm covering the intestinal and body -walls, and with the 

 lamellae of the primitive septa. This membrane constitutes the 

 dor>^al mesentery which persists throughout life and suspeuds the 

 intestine to the dorsal body-wall. 



Before considering in greater detail the characteristics of the 

 primitive septa, the tormation of the dorsal and ventral blood- 

 vessels may be briefly described, since these structures appear very 

 early and apparently play an important part in the nutrition of the 

 growing region. The dorsal blood-vessel is early formed as a spacc 

 between the opposed lamellae of the dorsal mesentery (Piate 2:'>. 

 Figs. 13, 20 and 21). It does not, however, appear to become weil 



