MOEPHOLOGY OF CYCLOSTOMATA. 313 



tures are distinguishable very clearly, being almost entirely dis- 

 colored in all parts except nuclei which are stained intensely. 



I wish here to express my warmest thanks to my former 

 teachers, Pkof. Miïsukuki and Prof. Ijima, for much in- 

 valuable advice and for their kindness in looking through the 

 manuscripts and the proof-sheets of this paper. 



To avoid confusion the present paper will be divided into tw^o 

 sections, the first of which will contain mere descriptions, while 

 in the second will be given a historical review^ and conclu- 

 sions. 



I. Descriptive. 



A. — The Pronephros. 



The youngest stage of the embryo which I have to deal 

 with in the present investigation, is only a little advanced beyond 

 an ellipsoidal gastrula ; it is intermediate between Stage I and 

 II of the list given in my first contribution above referred to 

 (fig. 1, A. and B, of that article). The head-fold forms a pointed 

 protuberance at one pole of the ellipsoid, while the blastopore 

 is plainly visible at the opposite pole. The prominent neural 

 ridge extends longitudinally from the anterior end of the head- 

 protuberance to the dorsal lip of the blastopore. 



For the general relation of the germinal layers at this stage, 

 I refer to fig. 1 which is drawn from the same series as the 

 section represented in fig. 18 of my former work.^^ It represents 

 a transverse section though the dorsal region of an embryo in 

 the stage above described. As seen in this figure, the solid 



1)S. Hatta, On tlie form, of the germ. lay. in Petr.: tliis Journal, vol. Y, 1S91. 



