96 EBEN J. CAREY 



This apparently continuous blastema! skeleton is in reality 

 segmental in nature (fig. 2). The segments appear progressively 

 from the proxmial to the distal end, much in the same manner 

 as that found in the successive caudal formation of metameres 

 in the chick embryo. In contradistinction to the idea that 

 segmentation of the continuous skeleton does not appear until 

 joint formation occurs, the evidence at hand proves that centers 

 of accelerated growth, segmentally arranged in the apparently 

 continuous blastemal skeleton are prior in time to the formation 

 of joints. This observation confirms that made by Bardeen 

 ('10) on human embryos as noted in the following statement: 

 "These cartilaginous anlages are embedded in a dense blastema 

 which shows lighter areas in the vicinity of the future joints." 

 The same fact is recorded by Schulin (79). 



With the relatively greater increase in width than in length 

 of the incipient femoral blastemal center, the peripheral cells of 

 this rapidly proliferating zone show signs of retarded growth. 

 They become elongated in the direction of the radial force of 

 growth, forming an encircling constricting periblastemal mem- 

 brane. With the formation of this limiting membrane, more 

 rapid growth in length subsequently ensues in embryos 14 to 

 25 mm. in length (figs. 3, 4, 5 and table 2). This is due to the 

 shifting of the planes of mitosis from a parallel to a transverse 

 position, as regards the long axis of the limb. 



The significant feature of the relatively and absolutely greater 

 femoral growth in length during this period is the concomitant 

 elongation of the nuclei of the premuscle masses in the direction 

 of the skeletal growth. There is a subsequent stretching of the 

 cytoplasmic myofibrils in the same direction. This inception 

 of muscle differentiation of the thigh is more than a mere coinci- 

 dence. It is an effect, a mechanical cellular response to the 

 traction or tension to which the syncytially continuous and 

 peripheral mesenchyme is subjected during the rapid growth of 

 the femur in length. It would be as impossible for the femur 

 to extend rapidly in length without elongating the continuous 

 surrounding mesenchyme as it would be to expand a rubber 

 balloon Vvithout stretching the limiting membrane. 



