IV.] 



THE BLOOD-VESSELS. 71 



same extent in all parts of the blastoderm. By far the greater part are formed 

 ill the vascular area, but some arise in tlie pellucid area, especiallj' in the hinder 

 part. In the front of the pellucid area the processes are longer and the network 

 accordingly more open ; the corpuscles also are both later in appearing and 

 less numerous when formed. 



The omphalo-mesaraic arteries and veins, and the sinus terniinalis which 

 from the first has a distinct wall, seem to take origin in a manner altogether 

 similar to that of the smaller vessels ; and the description of the formation of 

 the heart which we gave above shews that it too is nothing but a gigantic 

 nodal point. 



Assuming the truth of the above account, it is evident that the blood-vessels 

 of the chick do not arise as spaces or channels between adjacent cells of the 

 mesobiast, but are hollowed out in the communicating protoplasmic substance of 

 the cells themselves. It is also perhaps worthy of note that the red-blood 

 corpuscles are not cells, but nuclei. 



The red-blood corpuscles when removed from the vessels exhibit energetic 

 amoeboid movements. They seem to increase at this stage chiefly by division. 



The above is the view which we deduce from our own observations. The 

 following may serve as a brief summary of the history of the matter. 



Von Baer and the older embryologists regarded the blood-vessels as being 

 at first mere gaps or spaces between the cellular elements of the naesoblast, 

 hollowed out so to speak by the tiow of blood from tfie heart. The first steps 

 in the right direction were taken by Remak and Kolliker, who described the 

 formation of solid bands or cylinders composed of cells and arranged in a 

 close-set network. These bands, becoming hollowed by solution while their 

 central cells were converted into blood-corpuscles, gradually put on the 

 appearance of blood-vessels, the aggregation of the red corpuscles at various 

 points, through arrest of the circulation, giving rise to the blood-islands of Wolfl 

 and Pander. 



According to Afanassieff ( Pr?'en. Sit::. Bericht. Bd. 53, 1866) there appear 

 in the mesobiast vesicles of variable size, with protoplasmic envelopes and 

 contents. These vesicles, which are at first clear and homogeneous, subse- 

 quently become traversed with strands of nucleated protoplasm, forming often a 

 close net-work within the vesicle. The space intervening between the nume- 

 rous vesicles is cut up into a network of canals by protoplasmic processes 

 stretching from one vesicle to another. These canals are the rudimentary 

 blood-vessels. From the outside of the vesicles, forming the inner wall of the 

 adjacent vessels, nucleated masses of protoplasm are budded off as blood-cor- 

 pu.scles and fall into the current of the circulation. 



His (op. cit.), following out his peculiar theory of development, derived both 

 blood and blood-vessels from the white yolk or parablast. According to him 

 while certain of the white-yolk masses become converted into conglomerations 

 of cells, which acquiring a yellow colour stand out in surface views as blood- 

 islands, other white-yolk masses, metamorphosed into angular cells, form a 

 network of thick lines permeating the mass of true blastodermic (archiblastic) 

 cells of the mesobiast. These lines, at the first solid, subsequently become 

 hollow. The meshwork of canals, or rudimentary blood-vessels, thus developed 

 first in the vascular and pellucid areas and spreading thence into the embryo, 

 contains for a certain time clear fluid only, the blood-islands being imbedded 

 in or attached to the walls of the canal and surrounded by protoplasmic 

 envelopes, so that the blood-corpuscles are shut out from the cavities of the 

 vessels. Later on, however, the envelopes of the islands are broken through, 

 and the blood -corpuscles emerging from their nests fall into the current of the 

 circulation. 



His therefore regarded the blood- corpuscles as formed in greater part at least 



