566 



The process now about to be described for this embryo of 29 mm 

 goes on for a long time without abatement. For the present it will 

 be described in two embryos only, but it may be added that it is seen 

 in all embryos of 29 mm up to those of 42 mm. 



In what follows only those sections, w^hich lie well within the 

 thymus, and near its central portions, will be noticed. Tangential 

 sections near its anterior and posterior ends also exhibit the same 

 phenomena, but for obvious reasons it is better not to rely upon such. 

 The first section to be noted is the fifth out of ten (transverse to the 

 long axis of the embryo) passing through the first thymus-element of 

 the left side. The inner boundary of the thymus, destitute as yet of 

 any enclosing membrane, is intact and even except at two points. 

 At the one of these to the dorsal side there is a small break, and 

 one or two leucocytes are wandering out. At the other, which oc- 

 cupies a good portion of the lower inner surface, there is an exten- 

 sive break, and leucocytes are wandering out en masse. In the 

 mesoblastic region just beyond the thymus there are comparatively 

 few mesoblastic cells, but this space is occupied by great numbers of 

 leucocytes. 



In a single section upwards of a hundred of them can be counted. 

 Numbers of them lie closely along the wall ^) of the anterior cardinal 

 vein, which runs a little internal to the thymus. Many of them are 

 already in the blood itself, not only here in the section but in other 

 parts of the body also. A rapid infiltration of the blood and of the 

 mesoblast of all parts of the body is taking place. This process, in- 

 deed, began much earlier. A section (the fifth out of eleven) through 

 the second thymus-placode of the same side shows practically the same 

 things, but in the portion of the blood-vessel sectioned the leucocytes 

 are rather more numerous. 



These breaks for the emigration of leucocytes are very characteristic 

 of all the thymus-placodes (five on each side of the body) of all B aj a- 

 embryos from this stage up to embryos of 42 mm in length. As an 

 instance, the evidence afforded by the drawings of seven consecutive 

 sections through the second thymus-element of the right side from an 

 embryo of 33 mm may be cited. 



In the first section there is one break, in the second there are 

 two, in the third, fourth, fifth, sixth and seventh there is one large 



1) The tendenc}' on the part of leucocytes to attach themselves to 

 the walls of vessels aud capillaries is as characteristic of them in 

 embryonic life as later on. 



