HEART IX PETROMYZON. 227 



Stage VI I (Fig. IH) : — The external features are almost the same 

 as in the last srac^e, l")ut the dorsal un and a laro-e ]iart of the caudal 

 are developed ; the stomoda^um does not vet o[)en externally, and the 

 intestine is still crowded witli the yolk-cells. The embryo is about 

 6.5 mm. long and has seven pairs of open gill-slits and one blind pair. 

 One specimen had 7o pairs of the mesoblastic somites. 



Stage YIII : — The free swimming larva, having no longer any 

 yolk-mass ; the intestirje is tilled up with diatoms. It is ca. 9 mm. in 

 length and has about 97 pairs of the myotomes. 



In L\'tromyzi)n, the heart is found, independent of age, in the 

 same section of the body as the pronephros, the latter lyinii', in a 

 lateral surface view (Fig. 1 H.), just dorsal to the farmer. Moreover, 

 they are, during the earlier stage, of rhe same extent. The pro- 

 nephros arises a little earlier than the heart. As these two structures 

 appear ju.st in the ventral aspect of the thrcjat, where the slender head- 

 fold is connected with the larger posterior portion, the study of them, 

 in younger stages, is extremely difficult (see Figs. 3 and 5). Among 

 the same series of sections, these structures are cut, in some frontallv, 

 in others transversely, and in still others obliquely. To ascertain facts 

 concerning them, therefore, a number of series in various planes is 

 necessary. 



It will ]je well to begin this study with the stage when no trace of 

 the heart is yet to be detected ; in such a. stage is the embryo represented 

 in Fig. IC. In the angle which the crooked head-fold makes with the 

 expanded posterior portion, the fore-gut is, from early stages (7 

 Plate XIV, Fig. 27), expanded into a wide space. The lateral plates of 

 the mesoblast in this part grow downwards, from right and left, to 

 meet in the ventral median line. A little before this meeting takes 



