434 JOHN BEARD, 



by figures from the fifth, but the pressure of other, and to me now 

 far more interesting, work has prevented the carrying-out of this plan. 



Since in the earlier embryos, last described, considerable progress 

 has been made in the histogenesis of the thymus-elements, their later 

 history calls only for brief description. 



The general characters of a Raja batis embi-yo of 34 mm have 

 already been given, and the following account may be prefaced by a 

 few words as to the characters of embryos of 45 and 54 mm, and of 

 a young skate of 70 mm. 



Raja batis No. 237 measured 45 mm in the preserved condition 

 in 90 7o alcohol. The dermal sense organs of the head-region are 

 deploying from the neuro-epithelium, but they do not yet reach the 

 snout. Fibres of the eye-muscles are in formation, lens-fibres almost 

 fill the cavity of the lens, the retina is in part slightly pigmented, 

 the fibres of the optic nerve reach the brain, and there is a slight 

 chiasma. In the olfactory organ there is a double row of olfactory 

 folds, about 12 in each row. The naso-buccal groove is partly formed. 

 The semicircular canals are arising. There is a slight deposit of 

 cartilage on each side of the notochord in the head-region. Cartilage 

 is also in formation in the trunk around the notochord. There is a 

 distinct well-developed cartilaginous shoulder-girdle, and cartilaginous 

 basalia enter the fin. The stomach is formed. The spleen is indicated 

 as a slight projection of the mesentery. The pancreas is formed. The 

 germinal nidus projects inwards on each side. The Müllerian ducts 

 are partly formed. The cloaca is closed. 



Raja batis No. 245 measured 54 mm. The sense organs do not 

 yet reach the snout. Ampullary tubes are beginning to arise. The 

 retina is rather more pigmented, perhaps for a fifth of its extent. 

 The naso-buccal fold is established. The semicircular canals are al- 

 most complete. The upper and lower jaws are beginning to be carti- 

 laginous. In the trunk there are slight deposits of cartilage on each 

 side of the notochord, but these are not at four points as in Scyllium. 

 There are slight procartilaginous neural arches. In this, a potential 

 female, the Müllerian ducts reach far back. The spleen is still a mere 

 fold of the mesentery. The Wolffian ducts do not open into the cloacal 

 chamber, and the cloaca is closed. 



Raja batis No, 255 measured 70 mm. Ampullae are developing. 

 The eye-muscles are formed. The lens-cavity is almost obliterated, 

 there is an optic chiasma, and the optic stalk is almost solid. A 

 cartilaginous olfactory capsule is in course of origin. There are many 



