562 



With so extensive a series of embryos as mine it is a very easy 

 matter to determine a point, below which there are no leucocytes 

 whatever in the body. As a general rule, though this is apparently 

 not quite free from exceptions, it may be said that in embryos of 

 Raja batis there are no leucocytes in the blood or elsewhere in 

 embryos smaller than 14 millimetres. One case we have noted in 

 which a single leucocyte was detected in a thymus - element of a 

 10 millimetre embryo. This absence of leucocytes may even extend to 

 embryos of 17 mm, but in no embryo of 18 mm or upwards have I 

 failed to detect leucocytes (or cells taking on leucocytic characters) 

 in the thymus, as well as in the body and blood. 



To summarise the observations, it is the rule that leucocytes 

 should be everywhere absent, even in the thymus-placodes of embryos 

 of 14 mm and under. In specimens from 14 to 17 mm they may or 

 may not be present in the mesoblast, blood, and thymus. In all em- 

 bryos above 17 mm they will certainly be encountered in all these 

 places. The reason of this is simple. While the histogenesis of the 

 thymus may occasionally begin in a feeble fashion in embryos below 

 17 mm, it always sets in, if it has not done so earlier, in embryos 

 of 17—18 mm in length. 



I am thus obliged to set up a sort of neutral zone between two 

 periods. This is conditioned not by the poverty of the material but 

 by its richness. 



Experienced embryologists will readily admit the truth of this 

 statement. 



To illustrate it, let a single series of embryos be taken, one in 

 which there are apparently no duplicates. They may be twelve in 

 number and of the following sizes : 10, 11, 12, 13, 14, 15, 16, 17, 18, 

 19, 20, and 21 mm. Now, it is quite possible, that the examination 

 of such a single series as this, free from all duplicates, may reveal 

 an entire absence of leucocytes everywhere (in the thymus also), in 

 the first seven. The embryo of 16 mm may be as free of leucocytes 

 as that of 10 mm. The first leucocytes may be found in the embryo 

 of 17 mm, and they will also be present in all those of larger size. 

 Such finds would lead to the conclusion that the first leucocytes always 

 appeared on the scene in embryos of 17 mm, because — they had 

 once been first encountered in such an embryo. 



If several embryos of the sizes 14—17 mm were examined the 

 first origin of leucocytes would probably turn out to be more variable 

 than previously imagined, but it would remain a fact that there were 

 no leucocytes elsewhere, until some were present in the thymus. 



