DEVELOPMENT OF THE MAMMALIAN SPLEEN 281 



Mietens find free cells given ofT from the coelomic epithelium 

 which make their way into the underlying mesenchyme where 

 they proliferate and join the local mesenchyme in the formation 

 of the first rudiment of the organ. Toldt and Janosik derive the 

 rudiment exclusively from coelomic epithelium, while Laguesse 

 states that in Acanthias and teleosts the epithelium plays no direct 

 part in the formation of the rudiment, although he admits that 

 in earlier stages, before the rudiment has appeared, it contributes 

 cells to the underlying mesenchyme. 



MATERIAL AND METHODS 



'Embryos of the pig (Sus domesticus) were used as the chief 

 object of study in making this investigation. A series of embryos 

 beginning with those 7 mm. in length to stages near the end of 

 fetal life were collected and prepared for study. Also embryos 

 of the striped gopher (Citellus tridecemlineatus) , guinea-pig, 

 white rat, and sheep were employed for comparative purposes. 



On beginning the study of the origin of the spleen, it was found 

 to be expedient to begin with embryos of about 15 or 20 mm. in 

 length. The advantages of such a procedure are obvious; most 

 of the organs of the viscera have begun their development in an 

 embryo of that size, and, furthermore, the presence of the spleen 

 has been reported and figured by Minot, Tonkoff, and others. 

 Thus, in first making a superficial stud^ of such embryos, one 

 could familiarize himself with the general features of the embry- 

 onic structures and also develop a technique that would bring 

 out the best details of the organ in question. 



Various methods of fixation were employed, but Helly's Zen- 

 ker-formol fixative proved to be the most satisfactory. Embryos 

 up to those 12 mm. in length were left whole and placed directly 

 in the fixative. In those from 12 to 30 mm. slits were made in 

 the body wall, or the anterior and posterior portion of the embryo 

 was removed so as to allow the fixative to penetrate more rap- 

 idly. In all later stages, the embryos were dissected and the 

 embryonic stomach and spleen removed and placed directly in 

 the fixing fluid to insure rapid and thorough fixation. 



