The origin and histogenesis of the thymus in Raja batis. 435 



olfactory folds. In the hypophysis there is obvious budding. The 

 semicircular canals are complete, and a cartilaginous auditory capsule 

 is arising. Cartilaginous centra and cartilaginous neural arches are 

 in formation. The upper and lower jaws and the branchial arches 

 and their rays are cartilaginous. The degeneration of the transient 

 nervous apparatus is commencing. Trabeculae are forming in the 

 spleen. The first scales are in course of origin, and in the jaw there 

 is a dental ridge. The sex is announced by differentiation of the 

 testis. The cloaca is open for a few sections. 



In the cases last described it has been seen, that in embryos of 

 33 — 36 mm epithelial cells are restricted to the bases of the "placodes" 

 next to the gill-clefts. In an embryo of 37.5 mm (No. 219) still fewer 

 epithelial cells are present. The migration of leucocytes is not very 

 actively going on. There is as yet no capsule for each element and 

 no vessels enter it. 



In an embryo of 42 mm (No. 227) very much the like conditions 

 prevail, and in this instance the leucocytes in the thymus-elements 

 are evidently in active division. 



In Baja batis No. 229 (43 mm) the epithelium at the outer side 

 of each placode, i. e., that of the gill-cavity, is growing over the 

 point of attachment of the element as over a wound. No blood- 

 vessels yet penetrate the structure, but a connective tissue-capsule is 

 beginning to form. 



In Raja batis No. 237 (45 mm) the growth of the epithelial 

 membrane, above referred to, is complete, and, thus, for the first time 

 each element is shut ofi" from the gill-cleft, but it is still attached. 

 There are practically no epithelial cells left at the base of each ele- 

 ment, which now presents marked indications of lobulation. Connective 

 tissue is commencing to penetrate it ; and, while it as yet contains no 

 blood, one or two capillaries have formed around it. 



In Raja batis No. 245 (54 mm) each element is still attached by 

 a narrow stalk (Fig, 55). As shown in this figure, here and there in 

 the pharyngeal epithelium there are scattered leucocytes some little 

 distance from the thymus-element, but there is no evidence of their 

 origin here. Each element is washed by a network of blood-capillaries. 

 There is a well marked capsule, which here and there sends in a 

 septum. There are still no capillaries within the thymus. Emigration 

 has become more difficult, but there are plenty of leucocytes in the 

 mesh work of the capsule, and these wandering cells now seem to 

 make their way through this to reach the blood-capillaries. The septa 



28* 



