36 DEVELOPMENT AND GROWTH OF 



From the mesoblast forming at first only a small mass of 

 cells, which lies below the primitive streak, it soon comes to 

 be the most important layer of the blastoderm. Its growth 

 is effected by means of the formative cells. These cells are 

 generally not very numerous in an unincubated blastoderm, 

 but rapidly increase in numbers, probably by division ; at the 

 same time they travel round the edge of, and in some cases 

 through, the hypoblast, and then become converted in the 

 manner described into mesoblast cells. They act as carriers 

 of food from the white yolk to the mesoblast till, after the 

 formation of the vascular area, they are no longer necessary. 

 The numerous cases in which two nucleoli and even two nuclei 

 can be seen in one cell prove that the mesoblast cells also 

 increase by division. 



The growth of the hypoblast takes place in a very different 

 way. It occurs by a direct conversion, cell for cell, of the 

 white yolk spheres into hypoblast cells. This interpretation 

 of the appearances, which I will describe presently, was first 

 suggested to me by Dr Foster, from an examination of some 

 of my specimens of about thirty-six hours, prepared with silver 

 nitrate. Where there is no folding at the junction between the 

 pellucid and opaque areas, there seems to be a perfect continuity 

 in the silver markings and a gradual transition in the cells, from 

 what would be undoubtedly called white yolk spheres, to as 

 undoubted hypoblast cells (vide PI. I, fig. 5). In passing from 

 the opaque to the pellucid areas the number of white yolk- 

 spherules in each cell becomes less, but it is not till some way 

 into the pellucid area that they quite cease to be present. I at 

 first thought that this was merely due to the hypoblast cells 

 feeding on the white yolk sphericles, but the perfect continuity 

 of the cells, and the perfect gradation in passing from the white 

 yolk cells to the hypoblast, proves that the other interpretation is 

 the correct one, viz. that the white yolk spheres become directly 

 converted into the hypoblast cells. This is well shewn in 

 sections (vide PL I, fig. 4) taken from embryos of all ages 

 from the fifteenth to the thirty-sixth hour and onwards. But 

 it is, perhaps, most easily seen in embryos of about twenty 

 hours. In such an embryo there is a most perfect gradation : 

 the cells of the hypoblast become, as they approach the edge 



