Contributions to the Morphology of Cyclostomata 



I. On the Formation of the Heart in Petromyzon. 



by 

 S. Hatla. 



With Plate XVni. 



In 1891, I published a paper entitled " On the Formation of the 

 Germinal Layers in Petoromyzon " in this Journal, Vol. V. The 

 present article on the formation of the heart is the first of a series, 

 in which I propose to give the results of my study of later stages in 

 the development of that interesting Vertebrate. 



The mnterials and methods employed are quite the same as those 

 given in the above-mentioned paper. An addition to the materials has, 

 liowever, been made by a supply recently obtained at Kawagoye, a 

 county -town not far from Tokyo. 



As the time passed throuçrh bv the eg-o-s and larvce affords 

 no measure of the progress of the development taking place in 

 them, I give simple sketches (Fig. 1 A — H) showing the stages which 

 I distintruish in the icrowth of the animal from the frastrula. condition 

 to that of the complete (free-swimming) larva. 



Stage I. The gastrula : — The bod}^ has an ellipsoidal outline ; 

 the blastopore and the head-end are on the opposite poles of the ellipsoid. 

 Fig. 1 A represents the oldest gastrula in which the head-fold has 

 already become noticeable, being raised over the level of the blastoderm. 



