1890.] The Mesoderm and the Calom. 891 
Theories of the Mesoderm?—From the time of Von Baer’s 
Entwickelungsgeschichte, of which the first part appeared in 
1828, until 1868, when W. His’s great monograph on the chick, 
16, was published, embryologists recognized the three layers, and 
regarded the mesoderm as a natural unit. His led the way to 
our present conception by a little-known article, 75, on the mem- 
branes and cavities of the body, and his monograph, 76, above 
mentioned fully established the necessity of recognizing two main 
groups of mesodermic tissues. Accordingly he divided the 
mesoderm into two parts, the archiblastic* and paradlastic, 
corresponding respectively essentially * to mesothelium and 
and mesenchyma. Under archiblast, His included not only 
the mesothelial tissues proper, but also the smooth or organic 
musculature ; under parablast the mesenchymic tissue, except the 
smooth muscle. The terms used corresponded to his theory of 
the origin of the two parts of the mesoderm, for he believed that 
the archiblast arose in the axial region, and was contained in the 
embryo from the start, while the parablast arose peripherally, and 
grew in towards the embryo, a conception which was perhaps 
suggested by the appearance of the blood-vessels first outside the 
embryo proper. Seeking still further for the source of the sup- 
posed peripheral parablast, he believed he had found it in the 
germinal wall. The study of the relations of the wall in the 
chick induced him to think that the elements of the white yolk 
became parablast cells; moreover, the study of the hen’s ovary 
led him to the conclusion that the white yolk was developed from 
the granulosa cells, and that these cells arise from leucocytes. He 
thus traced back the parablastic cells to maternal leucocytes. It 
has been shown that the granulosa cells are not leucocytes, and that 
the granulosa cells do not enter the ovum ; the white yolk-grains 
never become cells, for it has been proved that all nuclei of the 
segmentating ovum come from previous nuclei, and lie in proto- 
plasm, not in the yolk-grains; and finally it has been shown in 
this chapter that the mesoderm arises, as a whole, not from double 
sources. Professor His’s views as to the origin of the parablast 
3 See ante p. 880. 
Du 1 ee | +h +, derm, entoderm, a hihiaeti A 
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