326 McMURRICH. [Vol. IV. 



from the endoderm cells in the vicinity, but do not show any 

 very decided differentiation into gland and nettle cells. The 

 older filament (Fig. 23), on the other hand, has numerous nettle 

 cells which stand out prominently in specimens stained with 

 haematoxylin, and gland cells are quite numerous. Upon each 

 side of the filament the endoderm cells are heightened so as to 

 form a very decided wing upon either side, and there is no trace 

 of any extension of the filament cells out upon this wing. The 

 appearance which is seen in Fig. 21 is due to the section having 

 passed through a bend of the filament. Owing to the contrac- 

 tion of the embryos the filaments are much contorted, and sec- 

 tions frequently give an appearance as if several filaments 

 branched out from the margin of the mesentery. A reconstruc- 

 tion in wax of a mesentery and its filament showed, however, 

 that this appearance was deceptive, and was due to the bending 

 and twisting of the filament. 



There is one bend, however, which is constant in all the 

 embryos, and occurs a short distance below the point where 

 the mesentery leaves the stomatodaeum. The edge of the 

 mesentery is here bent upon itself, so that transverse sections 

 show a portion of the mesentery attached to the stomatodaeum, 

 its edge being turned towards the edge of the rest of the mesen- 

 tery (Fig. 24). This bend is important since it is upon the por- 

 tion that lies morphologically above it, that the lateral streaks 

 of the filaments are developed. 



In the embryo from which Figs. 21 and 22 were taken, the 

 lateral streaks had developed upon the first and second pairs of 

 mesenteries. In these in a section taken just below the point 

 where the bend mentioned above occurs (Fig. 24), the median 

 streak {ms) of the filament is well differentiated, and the two 

 endodermic wings may be seen on that portion of the filament 

 which is below (morphologically speaking) the bend. On the 

 upper part of the mesentery, that which in the section is at- 

 tached to the stomatodaeum, quite a different structure is found. 

 The mesogloea of the mesentery here branches into three pro- 

 cesses at its free edge, and these support the three lobes of the 

 mesenterial filament. The two lateral lobes (Flimmerstreifen) 

 {Is) are readily to be made out from the dark stain which they 

 take, and on tracing them onwards towards the lower edge of 

 the stomatodaeum the epithelium which forms them can be seen 



