16 ORIGIN OF THE PULMONARY VESSELS IN THE CHICK. 



the embryo. With gentle heat the block is fixed upon a glass slide. In good 

 light, under the high power of a binocular microscope, holding a sharp scalpel 

 lightly in the fingers of the right hand, successive layers of paraffin are shaved off 

 until the overlying injected vessels are removed. The block is then reversed, 

 exposing the other side of the embryo, and the procedure repeated. The block is 

 then removed, the paraffin dissolved off in xylol, and the block mounted in balsam. 

 By this method the early pulmonary capillaries are brought out clearly and their 

 development can be readily followed. Figures 7 and 8 were drawn from dissec- 

 tions of this sort. 



Direct Dissection. This method was more practical with the older embryos. 

 The chick was injected as above described, dehydrated in graded alcohols, and 

 passed through benzine into oil of wintergreen. The preparation was then placed 

 under the high power of a binocular microscope, and held in position with a fine 

 camel's-hair brush. With needle-pointed forceps the large limb-buds on both 

 sides were carefully removed before attempting the more delicate structures. 

 The cardinal veins, duct of Cuvier, and sinus venosus were opened and brushed 

 free of ink granules. This procedure exposes the pulmonary vessels in situ while 

 preserving their anatomical relations. Such a technique was used for specimens 

 shown in figures 9 and 10. 



Serial Sections. In the early stages in which the splanchnic plexus can not 

 be injected, i. e., before the pre-pulmonary capillaries are patent, I resorted to 

 serial sections. The embryos were fixed with Bouin's mixture (75 parts picric 

 acid, 20 parts 40 per cent formalin, and 5 parts glacial acetic acid). After removal 

 from the shell the chick was fixed for one hour in this mixture, then passed 

 directly through several changes of 60 per cent alcohol to remove the excess of the 

 fixative, and finally through the graded alcohols to paraffin, as above described. 

 Sections 10 to 15 m in thickness were cut by the water-knife method of Huber and 

 stained in hematoxylin and erythrosin. Both sagittal and cross sections were used, 

 so as to serve as a check in either series and to give a more exact localization of the 

 anatomical structures. By using both types of sections the left valve of the sinus 

 venosus can be assigned to its correct position in relation to the mass of cells giving 

 rise to the common pulmonary vein. Sagittal sections have a close relation to the 

 injected specimens, which are used as guides. Erythrosin is used as a cytoplasmic 

 stain, although in the early stages the cells show a marked affinity for basic dyes. 

 Cochineal carmine may be used alone after cells have been identified. 



PULMONARY VEIN. 



The present status of our knowledge of the origin of the pulmonary vein is 

 embodied in the seemingly opposed views of Fedorow and Brown. The former 

 holds that the vein is derived from an endothelial proliferation of the dorsal wall 

 of the sinus venosus, while Brown thinks that it is a part of an indifferent plexus 

 originally present in this region. 



Fedorow (1910), studying embryos of four orders (amphibian, reptile, bird, 

 and mammal), reports the origin of the vein as an outgrowth of endothelium 

 from the dorsal wall of the sinus venosus. The cavity of the sinus extends into this 



