68 DEVELOPMENT OF PRIMITIVE BLOOD-VESSELS. 



of alcohol is by no means as marked as in the case of older embryos. The early 

 embryos are injured much more by maceration due to weak alcohol than by shrink- 

 age due to too rapid dehydration in the stronger grades. The specimens are all 

 cleared by the Spalteholz method of benzine followed by oil of wintergreen. Speci- 

 mens can be embedded from oil of wintergreen if they are passed directly from the 

 oil to a mixture of the oil and paraffin. The tissue does not become too brittle to 

 cut even after remaining in the oil for a year or two. 



VASCULAR SYSTEM OF THE CHICK. 



GENERAL ACCOUNT OF THE VASCULAR SYSTEM OF THE CHICK UP TO STAGE 



OF 14 SOMITES. 



In the study of the origin of the vessels of the chick I shall begin the detailed 

 account with the stage of 6 somites. The study of the blastoderm in a hanging-drop 

 preparation offers a valuable method for a study of the early stages. In the stages 

 of the early somites there is a plexus in the area opaqua which, by the older embry- 

 ologists, Pander, von Baer, Remak, and later by His, was identified as the fore- 

 runner of the blood-vessels. Basing his studies on those of von Baer and Remak, 

 His gave a description of the origin of blood-vessels which remains the foundation of 

 our knowledge upon this subject (1868, pages 95 to 100). He described the first 

 appearance of blood-vessels, or, as he later termed them, angioblasts, as occurring 

 just before the appearance of the somites. He stated that the vessels began as a 

 plexus of angular or spindle-shaped solid cells in the area opaqua. These cells from 

 the beginning were in the form of a plexus (Gleich von Anfang an ein geschlossenes 

 Mosaik, page 98). The plexus was at first made up of solid cells without a lumen, 

 and grew by processes of solid spindle-shaped cells, exactly similar to those which 

 formed the original network. This plexus was in a definite layer the vascular 

 layer (das Gefassblatt) of Pander. 



The vascular layer, His said, consisted not only of solid angular cells, but also of 

 elements having a yellow color, or, in other words, it gave rise not only to blood- 

 vessels but also to blood-cells. He regarded it as of the greatest importance that 

 the first appearance of vessels was in the area vasculosa before the heart formed, 

 and that these vessels arose entirely independently of any circulation. He then 

 noted that the plexus of solid cells became transformed into vessels, the exact 

 method of the transformation being impossible to determine; but that as the solid 

 cells became the walls of vessels, their cytoplasm became less granular and their 

 nuclei flatter. He then described the approach of the blood-vessels toward the 

 axis of the embryo by means of the same type of solid processes which formed the 

 original plexus and found that they approached the axis in two zones: first, oppo- 

 site the myotomes, and secondly, along the splanchnopleure in the region opposite 

 the future emphalo-mesenteric veins that is, over the ventral surface of the two 

 amnio-cardiac vesicles. 



His noted that over the region of these amnio-cardiac vesicles there was a 

 double sheet of vessels which approached the axis of the embryo, a more scanty sheet 



