438 DR THOMAS H. BRYCE ON 
A description of the external characters of the embryo and larva of Lepzdosiren and 
a general account of the affinities of the animal will be found in Professor GraHam 
Kerr’s original memoir in the Transactions of the Royal Society, B., vol. excii., and 
an account of the early stages of development in his paper in the Quarterly Journal of 
Microscopical Science, vol. xlv. part 1. 
The egg is markedly telolecithal, but the segmentation is holoblastic. The ex- 
cessive lading of the cells with yolk, and the consequent large dimensions of the 
primitive yolk cells, impresses characters on the development remarkable in certain 
respects. Among the secondary features due to this cause, the one that chiefly affects 
the present study is the long postponement of the formation of the alimentary canal. 
Hven at the stage numbered 31, the pharynx, gill clefts, and alimentary tract are 
solid, showing in no part an epithelial disposition of the cells. As a further con- 
sequence of this, the splanchnic mesenchyme is long in being differentiated. The 
splanchnic mesepithelium rests on a layer of smaller yolk-laden cells, in which blood- 
vessels form, but which is not separate from, or to be distinguished from, the general 
mass of the yolk cells. It is only when the alimentary canal commences to be differ- 
entiated that a layer of mesenchyme and a layer of definitive epithelial hypoblast cells 
can be distinguished. 
The general mesenchyme is differentiated much earlier. It arises, GRaHam KERR 
states, for the most part by a proliferation from the mesoderm, at about the level of 
the nephric rudiment, very much as in the Selachians, partly directly from the sub- 
notochordal region of the hypoblast. 
The history of the blood corpuscles may be divided into three phases. The first 
phase extends over the period from their origin up to stage 30, when the alimentary 
canal commences to be cut off. It is comcident with the laymg down of the heart and 
main vessels. While the phase lasts, the corpuscles, like the rest of the tissue cells, 
are laden with yolk; and though they vary much in character according to the 
amount of yolk borne, are at first all of one type, though very soon two kinds are to 
be distinguished. 
The second phase is coincident with the formation of the alimentary canal and 
the differentiation of the splanchnic mesenchyme. ‘The blood corpuscles, now free of 
yolk, like all the tissue cells save the hypoblast, are heterogeneous. There are 
erythroblasts, young erythrocytes in active division, and mature red cells, while, 
further, there are white elements belonging to different categories. The rudiment of 
the spleen appears at the beginning of this phase, and during its persistence is under- 
going its histogenetic changes. 
As the phase advances the erythrocytes become almost all of the mature variety, 
and the third phase is initiated, during which the permanent conditions are established. 
The great mass of the red cells are now mature, young corpuscles are sparsely dis- 
tributed, while the erythroblasts seen here and there in the vessels are densely crowded 
in the spleen pulp, and to a lesser degree in the venous sinuses of the kidney 
howl 
