432 JOHN BEARD, 



The process above described, of the emigration of leucocytes by 

 breaks in the placode, goes on for a long time without abatement. 

 Further details of it may be given in two further embryos, but it 

 may be added, that it is seen in all embryos of 29 to 42 mm. In 

 the following only those sections, which lie well within the placode, 

 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 placode of the left side. 

 The inner boundary of the placode, destitute as yet of any enclosing 

 membrane, is intact and even except at two points. At one of these 

 to the dorsal side there is a small break, and one or two leucocytes 

 are wandering out. At the other, which occupies a good portion of 

 the lower inner surface, there is an extensive break, and leucocytes 

 are wandering out en masse. In the mesoblastic region just beyond 

 the placode there are comparatively few mesoblastic cells, but this 

 space is occupied by great numbers of leucocytes, as in Figs. 40 

 and 41. 



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 placode. This tendency on 

 the part of leucocytes to attach themselves to the walls of vessels 

 and capillaries is as characteristic of them in embryonic life as later 

 on. 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, of course, began much earlier. A section, the 

 fifth of eleven, through the 2. 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 chracteristic 

 of all the ten placodes of all Raja-emhryos of 28—42 mm in length. 

 As an instance, the evidence alïorded by the drawings of seven con- 

 secutive sections through the 2. right placode of an embryo of 33 mm 

 (jR. batis No. 214), 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 break in each section, and in some of 

 them single leucocytes are emigrating here and there. 



