and Laboratory Methods. 1209 



made for the stages in which leucocytes first appeared in the blood and meso- 

 blast. Second, the development of the thymus was followed from its earliest 

 stages till the permanent characters appeared. The best material was found to 

 be that fixed in Rabl's picro-platino-chloride, or in corrosive sublimate. The 

 most satisfactory stain is picro-carmin. In this stain the red blood corpuscles 

 are yellow and the nuclei often unstained (in picric acid preparations) ; the 

 leucocytes are differentially stained, nuclei a brilliant red and their scanty proto- 

 plasm a yellowish-brown. All the sections were of embryos less than 30 mm. 

 in length and have been studied with the Zeiss 2 mm. apochromatic and a Leitz 

 ^ oil immersion. 



The origin of the thymus element is from a small area of modified epithelimn. 

 The term "placode" will be used for this thymus element, a name introduced 

 by von Kupper for such a small piece of modified epithelium. The thymus 

 elements arise in the skate as specialized portions of the epithelium of the gill 

 pouches before these open to the outside, and hence the thymus is of hypoblastic 

 origin. These placodes are five in number on each side. In most cases the 

 histogenesis of the thymus does not take place until the embryo is 17-18 mm. 

 long, leucocytes appearing at the same time. It is evident that the first of these 

 originate in the thymus epithelium, and until some are found there none are 

 present elsewhere in the mesoderm or blood. The first changes seen in the 

 transition of the epithelial cell into a leucocyte is an increase in the refractive 

 power of the cytoplasm and under favorable conditions it assumes a somewhat 

 brownish color. Gradually the nucleus changes from its oval shape and becomes 

 rounder, a form later assumed by the whole cell. Finally the nucleus comes to 

 its eccentric position. Many of these newly formed leucocytes wander out 

 immediately into the tissue, while others remain in the gland and increase by 

 division, as is shown by their being in groups of 2 and 4. Emigration of leuco- 

 cytes begins at first singly, but later the even contour of the gland is broken, one 

 break occupies nearly the whole lower surface, and here the leucocytes are 

 wandering out en masse. Many are here in the blood itself. These breaks in 

 the placode walls are very characteristic, all the thymus elements of all the 

 embryos from this stage up to those 42 mm. in length. 



The thymus of an embryo of 71 mm. has practically reached its adult 

 condition. The corpuscles of Hassall have never been seen, in the embryo, 

 young skate or adult. Their presence in fishes is uncertain, only one author 

 mentions finding them. The transition from the epithelial structure is as 

 follows : In an embryo of 33 mm. the epithelial cells are restricted to the basal 

 portion of each placode ; the emigration of leucocytes is in active progress ; no 

 blood vessels are as yet within the thymus and it is without a connective tissue 

 capsule. No spleen has yet been formed. In an embryo of 43 mm. a capsule 

 is in process of formation, but no blood vessels have formed. Connective tissues 

 strands are forcing their way in and lobulating the organ. In an embryo of 71 

 mm. the thymus elements are free from the epithelium of the clefts, separated by 

 the capsule growth ; this latter still permits the emigration of leucocytes, and 

 there are many such within the organ. Blood capillaries are now within the organ, 

 brought there by the connective tissue, and afford easy transport for the leucocytes. 



