HEART IN' PETROMYZOX. 229 



yolk-mass, the latter structure ( lower portion of the figure) appears 

 to be connected witli the former (upper portion of thefi^-ure). The gut 

 appears twice, viz. as the fore-gut in the neck portion and as the mid-gut 

 in the yolk-mass. Some sections further dorsad, these two cavities will 

 unite. The point to which I wish to call especial attention in Fig. 5 

 is the mass of mesenchymatous cells (x) lying between the fore-gut and 

 the mid-iiTit. It is without any reo-ular arranofement and is 

 hedged in on each side by a very much thickened portion (lu.sp.) of 

 the splanchnic plates consisting of a row of high columnar cells. The 

 thickening is limited to this small portion and ceases suddenly both 

 anteriorly and posteriorly. It will be seen further on, that this portion 

 plays an important rôle in the development of the heart, becoming, in 

 fact, the pericardial wall. 



By the rapid forward growth of the head-f)ld, the fore-gut is much 

 elongated, l)ut the mesenchymatous cell-mass, mentioned above, remains 

 behind, lying next to that part of the yolk-mass which is subsequently 

 transformed into the first section of the mid-o-ut. Fios. 6 and 7 

 are from a series of sections which ])assed somewhat obliquely in the 

 plane indicated by the line ah (Fig. 1 D') through an embryo of about 

 the same a<i-e as that fiu'ured in Fi"-. 1 D. The lateral i^lates of the 

 mesoblast are thinned out everywhere, excepting that part which encloses 

 the mesenchymatous cells. The latter are now arranged epithelially 

 and already show a lumen in the anterior part ; this is the first trace of 

 the ventricular endothelium, and the thickened p(_)rtion of the splanch- 

 nic plates is, therefore, the pericardial coat. 



Figs. (S, 9, and 10 are selected from the serial sections of the next 

 advanced stage (Fig. 1 E). On the right side of Fig. 9 is seen the 

 longitudinal section of the second pair of the pronephric tubules. The 

 endothelium (end) encloses the lumen of the heart as a complete layer 

 which is, in its turn, surrounded by the thick pericardial coat. 



