DEVELOPMENT OF THE BLOOD VESSELS IN THE FROG. 203 



but at the two ends, and especially posteriorly, it is still a mere 

 lacunar space in the mesohlast. 



From the outer side of each aorta three diverticula are given off. 

 The first of these passes into the hyoid arch, and is considerably 

 larger than the aorta itself at this level; the second is rather smaller 

 and passes into the first branchial arch ; and the third diverticulum 

 passes into the second branchial arch. 



In addition to these diverticula of the aorta, small lacunar spaces 

 are present in the dorsal part of the hyoid arch ; a series of irregular 

 detached lacunas occur along the whole length of the first branchial 

 arch, and a few small lacunar spaces near the dorsal end of the 

 second branchial arch. 



c. The Mode of Formation of the Blood Vessels.- — It is difficult to deter- 

 mine the precise mode in which the blood vessels arise. In the first 

 instance, as in the first appearance of the aorta, or of the efferent 

 vessel of a branchial arch, a number of irregular and independent 

 lacunar spaces are seen, which are apparently merely local enlarge- 

 ments of the intercellular spaces present everywhere in the mesoblast. 

 These spaces rapidly enlarge, and open into one another ; the meso- 

 blast cells bounding them soon acquire distinctive characters ; they 

 become arranged more closely and more regularly ; they stain more 

 deeply than the other mesoblast cells, and they soon form a definite 

 wall to the vessel. 



The further growth of the vessel may either be effected by a 

 continuance of this process, new lacunae arising independently in the 

 mesoblast, and then opening out into one another and into the already 

 formed vessel ; or it may take place in a different fashion, the meso- 

 blast cells becoming arranged in compact strings continuing the lines 

 of the already formed vessels, which strings are at first solid, but 

 become tubular by the breaking down of their central or axial cells, 

 and so become vessels. This second mode of growth appears to be 

 the more usual ; it is probably to be regarded as a modified form of 

 the first method. 



Blood corpuscles are at first absent; they, however, soon appear, 

 though in small numbers. They form in situ in all parts of the body, 

 and may be seen in lacunae which still have no connection with 

 other vessels. They are apparently in all cases individual mesoblast 

 cells, and are at any rate in many cases formed by division of tho 



