June, 1916] Epibranchial Placodes of Squalus Acanthias 343 



the granular appearance are in evidence throughout. There is 

 evidence of active proHferation in the placode, though meta- 

 ;morphosis is evidently not proceeding so rapidly, since the 

 mass of metamorphosing cells is quite large in proportion to the 

 size of the mass of undifferentiated cells. There is no distinct 

 boundary line between the placodal and general visceral com- 

 ponents, showing that fusion between the two is quite complete. 

 The placodal mass does not persist to the posterior extremity 

 of the ganglion but seems to occupy a position about the middle 

 of this structure. 



In the fourth epibranchial of division X, the form and 

 position of the ganglionic mass are such as to make the con- 

 tributed mass probably appear larger than it actually is. The 

 length of the ganglion is 210 microns and that of the contact 

 area is 180 microns, giving a ratio of 1-1.4 between the contact 

 area and the total length (Table I). In the 18 mm. embryo, 

 the length of the contact area is 100 microns. This is further 

 evidence of a lesser degree of maturity in the more posterior 

 ganglia. 



The point of contact lies dorso-mesial to the middle of the 

 external aperture of the gill cleft (Fig. 42 to 50). The placode 

 is easily distinguishable from the other ectodermal thickenings 

 throughout the entire length of the ganglion (Fig. 43, 44, 45, 

 47 and 48). The general visceral portion of the ganglion does 

 not maintain its outline after contact with the mass of contrib- 

 uted cells and there is such complete fusion between the two 

 masses that a definite boundary is not discernable except in 

 Fig. 49. The presence of mitotic figures, the complete fusion 

 between the general visceral and placodal masses and the 

 large size of the latter, indicate very rapid proliferation, while 

 the large size of the mass of incompletely metamorphosed cells 

 as compared to the size of the mass of undifferentiated cells, 

 indicates comparatively slow metamorphosis. 



In Fig. 45 and 46 there may be seen a constriction in the 

 mass of contributed cells which later results in complete sep- 

 aration between the ganglion and the placode (Fig. 47, 48, 49 

 and 50), leaving a large mass of cells attached to the placode. 

 This mass, in some sections, shows a tendency to lamellation 

 (Fig. 49 and 50). Posterior to the point of complete separation 

 of the ganglion from the placode, the contributed mass is rel- 

 atively much smaller than the general visceral portion. 



