30 The Development of the '’hymus 
All pig material was fixed in Zenker’s fluid, embedded in paraffin, and 
mounted in serial sections from 3» to 10 thick. Except those of the 
young stages (8 mm. to 27 mm.) the sections were stained on the slide. 
Most of them were stained with hematoxylin or iron-hematoxylin and 
counterstained with Congo red. For special purposes many other stains 
were used. 
To demonstrate the delicate protoplasmic threads of the syncytium 
during the later stages of the lymphoid transformation, I stained by the 
iron-hematoxylin method but omitted the final decolorization in iron- 
alum. Protoplasm is stained deep black; nuclear structure is poorly 
shown, but the finest cytoplasmic processes may be seen. 
For the demonstration of connective tissue fibrille in the syncytium, 
I found the method recommended by Jackson (13, 8. 39) most satis- 
factory. 
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Midis, LMie'5 | Ik Abioy-Ge UE 2. 
Text Figure 1. Cranial view of third gill pouch (thymic anlage); X 38; 
pig embryo, 11 mm.; ec, ectoderm; 7, lumen; nt, nodulus thymicus; ph, con- 
nection to pharynx; sp, sinus precervicalis. 
TEXT FIGURE 2. Ventral view of thymic anlage; X 33; pig embryo, 15 mm.; 
ec, ectoderm; 7, lumen; nt, nodulus thymicus; ph, connection to pharynx; 
Sp, Sinus precervicalis. 
To determine the relation of the blood-vessels to the corpuscles of 
Hassall, I put a young kitten under deep anesthesia and injected a large 
quantity of a strong aqueous solution of Prussian blue into the aorta 
through the common carotid artery. ‘The heart continues to beat even 
after an amount of fluid twice as great as the total volume of the blood 
has been injected. An injection made in this way is under a slightly 
increased blood pressure and easily reaches the finest capillaries. There 
is therefore a thorough injection with little danger of rupturing delicate 
blood-vessels. 
ORGANOGENESIS. 
My observations on this phase of development are not sufficiently com- 
plete to warrant a full discussion. <A brief survey may however prepare 
the way for a better understanding of the histogenesis. The text figures 
show the shape in outline of the third gill pouch and thymic anlage 
