Plate VII. 

 Later Embryos. 



(Preparations magnified about 25 diameters.) 



of. Otic vesicle. 



ol. o. External opening of otic vesicle. 



p. Pineal outgrowth. 



f. a. g. Postanal gut. 



/«. Pronephros. 



fn. d. Pronephric duct. 



f. f. Pectoral fin. 



s. Spiracle. 



s. z'. Sinus venosus. 



z'. Ventricle. 



Fig. 42. — Embryo, age about 25 days, corresponding to Balfour's stage G (+) of shark. This 

 embryo bent during the process of fixation. It shows especially well the knob-like out- 

 growth, k. in the region of the forebrain. 



Fig. 42 (7 and /'. — Anterior region of preceding embryo. Shown in nearly lateral and in dorsal 

 aspect. 



Fig. 43. — Embryo, age about 29 days, corresponding approximately to Balfour's stage i in shark. 



Fig. 43 b. — Anterior region of specimen similar to preceding. 



Fig. 44. — Embryo, age about 31 days, corresponding approximately to Balfour's stage j in shark. 

 It shows a bulbous caudal thickening. 



Fig. 45. — Embryo, age about 40 days, corresponding approximately to Balfour's stage k in shark. 

 The circular area under the letters g.' g." was found to be artifact. 



Fig. 46. — Embryo, age about 45 days, somewhat more advanced than Balfour's stage l in shark. 

 At the time of fixation the embryo probably- twisted, so that its axis came to lie nearly 

 parallel to the neighboring margin of the blastoderm. (Length of embryo 20 mm. ) 



186 



