HEAET IN PETROMYZOX. 231 



free. I have en den von red to show the process by the series of sections 

 represented in Figs. 12, 13, and 14. In Fig-. 13 the second pair of 

 the pronephric tubules is cut through. The endothelial lining and the 

 pericardial coat much resemble those in the former stage, but, by this 

 time, the latter has lost all its connection Avith the pleuro-peritoneal 

 membrane. The body cavity now ceases to be in separated chambers, its 

 right and left halves uniting together (p. p. c). This section passes 

 through the ventricular portion of the heart. Fig. 12 represents the 

 fburtli section anterior to Fig. 1 3, and passes through between the 

 first pair oftlie jjronephric tubules and the posterior end of the brancliial 

 chamljer. The s|»lanc]Hiic plates are comparatively thick at every 

 point, and the mesocardmm posterius still holds its place, while the 

 vicsocardiwn (interius has disappeared ; this section corresponds with 

 the iiiiNcits aiieyiosus. Fig. 14 represents the sixth section posterior to 

 Fig. 13 ; the third pair of pronepliric tubules are cut througli ; the 

 funnels come into view in a bitaterally symmetrical manner. The 

 heart is just as in the former stage : both the mesocanliuiii atiterius and 

 the 111. posterius (inc. a. and nie. p.) are present. The section is througl» 

 the liindmost part of the heart, constituting the auricle and the 

 sinus renosus. 



Increasing in its length and breadth, the heart in the next 

 older stage is constricted in the middle, being thus divided into 

 conical halves, whose a|)ices are turned respectively forwards and 

 backwards. In this constriction the endothelial lining is of course 

 involved, and thus comes to enclose two cavities connected together by 

 a small aperture. The anterior chamber is a little larger than the 

 posterior and constitutes the ventricle and the truncus arteriosus while 

 the posterior forms the auricle and the sinus venosus. The aperture 

 between them is no other than the auriculo-ventricular passage. Fig. 

 15 (from the stage corresponding with that of Fig. 1 H) passes through 



