380 Ralph S. Lillie 



at the zoue wliere least resistance is offered to the passage of blood 

 betweeii the two vessels, and tlius represcuts no sharply defined or 

 persistent Channel. The branchìng blood-sinus which in hiter hfe 

 envelopes the stomach-region seeins to be fornied in essentìally the 

 same manner. 



The primitive septa are formed, as above described, by the 

 apposition of the adjoining walls of two successive mesoblastic 

 somites. The walls of these earh' somites are, however, never 

 formed of a regulär epithelial layer of well-defined cells, but to ali 

 appcarauce consist of a continuous syncytial layer of protoplasm eon- 

 taining numerous nuclei (Piate 23, Figs. 13, 14, 23, 24), and closely 

 applied to the surface of adjoining structures. Each nucleus may 

 for puri)0scs of description be regarded as belonging to a single 

 celi, but definite celi- walls, at this stage at least, are never 

 distinguishable. • 



The striicture of a septura at its earliest distinguishable phase 

 may be best seeu in Figs. 9 and 10, Piate 22 and 23, and Fig. 31, 

 Piate 23. At this period of its formation the septum forms an irre- 

 gulär nucleated partition, separating the cavity of the newly-forming 

 somite froni that of the soniite next in advance. At this stage no 

 bilaminar structure can with certaint}' be discerued in it, and the 

 constituent cells, which resemble in ali respects the other mesodermal 

 cells of this region, seem to be entirely undifferentiated and similar 

 to one another. As growth proceeds, however, the coelomic cavities 

 increase in size and in transverse diameter in the manner described 

 on page 374; the septa are thus subjected to a stretching process 

 and increase rapidly in superficial extent, becoming at the same 

 time thiuner and better defined. During this process the opposed 

 faces of the cells of the two septal layers (which are now usually 

 distinguishable from each other) assume a more membranous character, 

 becoming at the same time more dceply staining and apparently of 

 a denser consistency (Piate 24, Fig. 32). As the septum continues 

 to exteud it acquìres more and more of the character of a thin 

 membrane, and the nuclei of its constituent cells are moved farther 

 and farther a\K\ìt. Finally the septum acquires the appearauce oÌ 

 a thin, sharply defined, membranous partition extcnding across the 

 body cavity, and to whose surface the originai nuclei (each of which 

 is stili surrounded by a small quantity of undifferentiated ])rotoplasm) 

 are ajìplied at infrequent intervals (Plates 22—24, Figs. 9, 29—34). 



At the Junction of the septum with the body- wall and with the 



