462 . DR THOMAS H. BRYCE ON 
ringed yolk-free corpuscle with a round nucleus; (b) small cells with a small amount of 
protoplasm formed (?) by budding (unequal division) of larger yolk-laden cells, and (c) 
similar cells with slight concentric fibrillation of the protoplasm. 
(5) In all stages up to 30 the nucleus in every variety of corpuscle is identical 
except in size. In the arrangement of the chromatin it agrees with that of the large 
mononuclear basophile elements of all later phases. 
(6) The cells from which the smaller elements are derived are located certainly in 
the mass of yolk cells under the splanchnopleuric mesepithelium, possibly in the somatic 
mesenchyme in the neighbourhood of the developing cardinal veins. 
(7) From stage 30 onwards the active cells appear in progressively greater numbers 
up to stage 32+, in which they are very plentiful. 
(8) From stage 30 onwards there are always cells with nuclei of the erythroblast 
type, and fine concentric fibrillation of a basophile protoplasm. Side by side with these - 
are hyaline cells with small amount of basophile protoplasm and simple nuclei of the 
leucocyte type, identical with the nuclei of the earlier young red cells. 
(9) The mass of yolk cells immediately under the splanchnopleuric mesepithelium 
begins at stage 30 to become differentiated with the growth of the liver, and isolation 
of the gut rudiment, into definitive hypoblast and splanchnic mesenchyme. 
(10) At stage 31 the splanchnic mesenchyme is a cellular tissue with spaces 
containing blood corpuscles and free elements of two kinds: (qa) cells identical with 
the large mononuclear cells appearing in increasing numbers in the blood; and (b) 
polymorphonuclear cells, with either hyaline or granular protoplasm. The nephric tract 
of somatic mesenchyme also contains a few free mononuclear and polymorphonuclear 
cells, but in smaller numbers than occur in the very cellular splanchnic mesenchyme. 
Mitoses are frequent in both tissues. 
(11) At stages 32 and 32+ the splanchnic mesenchyme holds in its spaces numbers 
of mononuclear basophile cells, and in parts is composed almost wholly of polymorpho- 
nuclear cells, with hyaline or granular protoplasm. The whole perinephritic tissue has, 7 
massed in the spaces between its stellate cells, very large numbers of mononuclear 
basophile cells, with nuclei becoming transformed into every grade of lobing, and their 
protoplasm acquiring every degree of granulation. The portal and hepatic veins, and 
the cardinal vein and spaces communicating with it, contain white cells of different 
categories, the mononuclear preponderating. The cardinal vein contains three times as 
many erythrocytes as the aorta, but seven times as many white elements. The 
mononuclear outnumber the polymorphonuclear by four to one in the cardinal vein, 
while in the aorta they occur practically in equal proportions. 
(12) The splanchnic mesenchyme retains its primitive characters in part, round the 
gut along its whole length, forming the so-called lymphoid tissue of the adult. 
(13) The nephric tract of the somatic mesenchyme also retains its primitive 
characters and forms the so-called lymphoid tissue of the adult kidney. 
(14) The spleen rudiment is a tract of, at first identical, closely packed mesenchyme 
