440 JOHN BEARD, 



even more prominent in the next figure, Fig. 62. This is from an 

 embryo of 27 mm. The simple epithelium of Fig. 61 is now becoming 

 more complex, in that increase in thickness is taking place, and greater 

 numbers of leucocytes are in formation. There are more leucocytes 

 within the epithelium, and in each section of the placode there are 

 from four to six in the mesoderm to its inner side. Very similar 

 conditions are met with in Fig. 63 from an embryo of 28.25 mm. 

 From this epoch up to embryos of 54 mm there are no figures in 

 the plates, because the placode for another prolonged period under- 

 goes little change, beyond increase in area. 



In embryos of 42 to 45 mm the leucocytes within the epithelium 

 are more numerous than in embryos of 27 to 28 mm, but they are 

 still not abundant. The epithelium itself is hardly more complex, but 

 its area is greater, a change furnishing a key-note to a point in its 

 later history. In an embryo of 54 mm (No. 245), as depicted in 

 Fig. 65, the spiracular thymus is not more complex as a whole than 

 the thymus-placode of the first branchial cleft of an embryo of half 

 the size. As the figure reveals, it has fourfold the thickness of the 

 simple epithelial spiracular thymus of a 27 mm embryo, but there 

 are not four definite layers of epithelial cells. Its inner and outer 

 sides are bounded by well-marked epithelial layers, while its interior 

 is filled by epithelial cells, lying in all sorts of directions, and here 

 and there a single leucocyte, or a small group of such, is seen. In 

 addition, as in the other thymus-elements of this period and size, 

 there are blood-capillaries (Fig. 65 v). 



To this period, and from another embryo only 2 mm larger, 

 belongs Fig. 66. Whereas the former figure was from a series of 

 transverse sections, the present drawing is taken from a series of 

 frontal (horizontal) sections. The placode is thicker — nearly as 

 thick again — but at the same time more circumscribed. Here the 

 increase in area was evidently lacking. It is bounded everywhere by 

 epithelial cells, but in its interior such epithelial elements are entirely 

 wanting. This is made up of leucocytes, and here and there con- 

 nective tissue trabeculae and blood-capillaries are met with, as in the 

 other thymus-elements of this period. 



A complete contrast is afforded by the conditions in a young 

 skate of 7 cm (No. 255), and this vividly recalls Van Bemmelen's 

 description ^) of the spiracular follicle. In this case on both sides of 



1) In several Scyllium canicula of 33 — 39 mm I have found a 

 spiracular follicle, exactly like that described by Van Bemmelen. It 



