HUMAN MAGMA RETICULE IX NORMAL AND PATHOLOGICAL DEVELOPMENT. 9 



delicate bands of fibrils, among which appear scattered nuclei. Near the embryo 

 there is a small space, the interpretation of which was very difficult at the time the 

 specimen was described. 



Since Peters studied this specimen, the sections have been carefully reworked 

 and discussed in a critical way by Grosser, who gives a new interpretation in two 

 figures and states that the cavity of the ovum contains reticular magma which is 

 ])artly made up of heavier strands of tissue accompanied by nuclei. In the neigh- 

 borhood of the embryo there are two large spaces, lined with colls, which appear 

 to be the primitive bodj'-cavities. In his work on the comparative development 

 of the embryonic membranes, Grosser describes this space in great detail and also 

 gives us two new illustrations of the embrj-o m his plates 3 and 4. According to 

 this authority these two body-cavities comnnmicate by means of a slit-like canal 

 just behind the umbilical vesicle (Grosser's figure 31, plate 4). This interpretation 

 of the Peters specimen shows that the cavity of the ovum is first filled with a free 

 mass of reticular magma, after which the coelom begins to form near the body of 

 the embryo. As this cavity exj^ands subsequently, it probabh' first destroj's the 

 more delicate strands of magma, leaving the heavier ones; thus in a short time the 

 cavity of the ovum is lined b}' the endothelium of the ccelom, which also must cover 

 the stronger bands of magma radiating as trabeculse throughout this cavity (Grosser, 

 pp. 78 and 79). 



Keibel explains the formation of the human coelom as follows : 



"It is, however, not quite clear how the cavity traversed by scattered strands of niesohlast and 

 lying between the yolk-sac and the chorion in the Peters ovum is to be interpreted. It may be sup- 

 posed to represent the extraenabryonic ca'lom; but it may also be imagined that it has arisen from 

 an extensive loosening up of ths tissue, and not by a splitting of ths mssoderm, and that the triangular 

 space between the caudal extremity of the embryo, which is lined with flat cells having an epithelial 

 arrangement, is the first primordium of the coelom." 



A condition similar to that found in the Peters specimen has been observed by 

 Lewis in the Herzog specimen, which is of about the same stage of development. 

 Lewis says (see his paper, p. 300) that there are occasional clefts in the mesoderm of 

 the chorion of the Herzog embryo, but that they are of doubtful significance. His 

 reconstruction shows a strand of mesoderm, more pronounced than in the Peters 

 ovum, extending from the yolk-sac to the chorion and circumscriljing a space on the 

 ventral side of the embryo. 



Eternod has written several papers in which he describes the formation of the 

 exocoelom and the fate of the magma reticule. He says that it first fills the entire 

 space between the primordium of tire embryo and the chorionic wall. Later, larger 

 spaces appear within the substance of the magma, leaving denser strands of magma 

 fibrils to support the embryo within the gradually expanding chorion. In general 

 this coincides with the opinions just cited. 



The relation of the exoccelom to the magma is strikhigh' shown by Waterston 

 in a section of a small embryo in situ. The space between the embryo and the 

 chorion is filled with a dense mass of fibrils, into which the exocoelom is burrowing. 

 Waterston's figure 1 shows the relation of this cavity to the magma, and only near 

 the embryo is the exoccelom lined with a layer of cells. When this figure is compared 



