UMBILICAL CORD OF THE -PIG 5 



twelve cells occupy the diameter of each of the two elements. 

 Most of the central cells appear hollow, the nuclei having become 

 moved to one side, almost invariably to that side nearest the 

 center of the structure. These cells have a superficial resem- 

 blance to fat cells. The hollow condition is probably the result 

 of the coagulative action of the fixing fluid upon the delicate 

 cytoplasm. A few of the cells appear keratized, and are acido- 

 philic in staining reaction, and solid. The nuclei of the outer- 

 most cuboidal cells are deep-staining, granular, greatly elongated 

 bodies. Those of the more central cells have a generally irregu- 

 lar shape with wrinkled contour and a generally non-granular 

 homogeneous nucleoplasm. No mitotic or amitotic figures can be 

 detected. Each member of the double structure is enveloped by 

 a very thin inner connective-tissue theca, forming a delicate, 

 fibrillar, nucleated basement membrane. Both are inclosed in 

 a common, more external theca. The intervening partition only 

 consists of the fused basement membranes. 



The fact that four of the five specimens of human cords used 

 in connection with this study also contain a double yolk-stalk 

 indicates that this doubling is a common condition. Two ex- 

 planations suggest themselves: 1) That the doubling is due to 

 the partition of the originally single stalk by the ingrowth of a 

 connective-tissue septum related to the regressive changes by 

 which the stalk becomes obliterated. 2) That the 'doubling' 

 is only apparent, it being due in section to a sharp turning of 

 the stalk in certain regions. The latter interpretation is sup- 

 ported by the evidence from one of the human cords: here one 

 of the 'halves' is cut transversely, the other half is very 

 obliquely cut, and the two are in continuity. In other words, 

 the condition is such as would result if the section passed ob- 

 liquely through the proximal end of one of the limbs and the 

 connecting loop of a U-shaped structure. Opposed to the latter 

 interpretation, however, is the fact that in the specimen of the 

 pig's cord here described a common connective-tissue sheath 

 envelops both moieties. 



