72 THE SECOND DAY. [CHAP. 



separately from the blood-vessels ; their entrance into the vascular spaces being 

 an after event. The parablastic cells (derived from the white yolk), in his view, 

 give rise to the epithelium (endothelium) and connective tissue elements only 

 of the blood vessels, the muscular elements being derived from genuine (blasto- 

 dermic) mesoblastic cells. 



Klein (Wien. Sitz. Bericht. LXIII. 1871) describes the blood-vessels as 

 taking their origin from certain cells of the mesoblast in which a vacuole, 

 appearing and rapidly increasing in size, pushes the nucleus on one side, 

 leaving only a thin layer of protoplasm round the periphery of the cell. In this 

 thin layer nuclei appear; and, multiplying, form a complete nucleated invest- 

 ment, to the vacuole, which meanwhile continues to increase in size. From the 

 inside of this protoplasmic investment cells are budded off, and fall into the 

 vacuole. Here they soon acquire a red colour and become converted into 

 blood-corpuscles. From the exterior of these vacuolated cells nucleated 

 processes are thrown out, which end freely or join with similar processes from 

 other cells. A protoplasmic network is thus formed, the lines of which become 

 vacuolated, and hollow, and ultimately communicate with the original central 

 vacuoles now crowded with corpuscles. By these means a system of com- 

 municating tubes is established. Klein also describes two other forms of 

 cells somewhat differing from the above, but also taking part in the formation 

 of the blood-vessels. One of these forms is found chiefly in the vascular area, 

 and he believes that these latter are simply the formative cells of which we 

 have already so often spoken. 



It will thus be seen that Klein's view, from which our own differs chiefly in 

 reference to the matter of vacuolation, is a return, with some modifications and 

 extensions, to the earlier view of Rernak, and that the accounts of both Afanas- 

 sieff and His, which in turn agree in many respects, have proved to be uncorro- 

 borated divergences from the older track. 



Still more recently Goette (Archiv fur Micro. Anat. Vol. X. 1873, pp. 

 145 199) has given an entirely different account of the origin of the blood- 

 vessels and bloo<l -corpuscles in the vascular area. He believes that in the 

 thick mass of cells immediately outside the 'pellucid area' (vide Chap. in. 12) 

 a quantity of fluid collects and causes the cells to separate into a network with 

 large spaces filled with fluid. Into these spaces the formative cells travel, and 

 undergoing a species of endogenous cell-formation, form masses of bl/od- 

 corpuscles the blood-islands of the ear.ier authors. This view differs, it will 

 be seen, from all the later views, and goes back to that of Von Baer in regard- 

 ing the blood-vessels as primitively mere gaps between the cellular elements. 

 In the investigation of such a point as this, sections (which apparently Goette 

 has alone employed) are very untrustworthy. 



7. The cells of the epiblast and hypoblast as well as of the mesoblast 

 undergo considerable changes between the 24th and the 36th hour. 



Up to the 24th hour the cells of both layers, but more especially of the 

 hypoblast, were filled with fine granules and also contained many highly 

 refractive spherules. By the 36th hour, however, they have become much 

 more transparent. Each cell now consists of a clear protoplasm with hardly 

 any granules or spherules, and a large oval nucleus together with one or more 

 vacuoles is distinctly visible. 



The cells of the hypoblast still pass insensibly into the white-yolk cells ; 

 and it is still by the conversion of the white yolk into hypoblast that the 

 peripheral extension of the latter is chiefly carried on. 



The hypoblast cells beneath and at the sides of the embryo are markedly 

 smaller than those at the periphery of the pellucid area. 



The epiblast cells exhibit considerable variation in size in different parts of 



