164 Development of Tlioracic Ycrtcbne in Man 



The Blastemal Period. 



The division of the axial mesenchyme into segments, sclerotomes, 

 which correspond to the myotomes and spinal ganglia, is marked at an 

 early stage by intersegmental arteries. Schultze, 96, has shown that 

 the segmental differentiation of the axial mesenchyme extends into the 

 region dorsal to the spinal cord. Ventrally it does not, however, extend 

 quite to the chorda dorsalis. Fig. 1, Plate I, illustrates the conditions 

 existing in the thoracic region of man at this period. 



V. Ebner, 88, found in the embryos of several vertebrates a fissure 

 which divides each sclerotome into an anterior and a posterior portion. 

 Schultze, in 1896, showed, that in selachians and reptiles this fissure is 

 represented from the time of its formation by a diverticulum which 

 communicates with the myoccel. In birds the diverticulum arises sec- 

 ondarily and later becomes fused with the myocoel, and in mammals it 

 arises after the myocoel has disappeared. 



In man the fissure becomes distinct in the thoracic region at about the 

 end of the third week of development (Fig. 2, Plate 1).' At this period 

 the median surface of each myotome has become converted into muscle 

 fibres (Fig. 2, Myo.). At the same time the mesenchyme in the postero- 

 lateral region of each sclerotome has become condensed so that it ap- 

 pears, in a stained section, dark when compared with that of the anterior 

 half (Fig. 2). At the lateral margin of the anterior halves of the 

 sclerotomes the spinal nerves expend out toward the thoracic wall (Fig. 

 2, 8p.N.). The division between the sclerotomes is still marked by the 

 intersegmental arteries (Fig. 2). About the chorda dorsalis the cells of 

 the axial mesenchyme become densely grouped into a perichordal sheath. 

 The long axes of the cells lie parallel with the chorda (Fig. 2, Pch. 8.). 



The condensation of tissue which distinguishes the posterior sclerotome 

 half begins, as mentioned above, in the posterior lateral area of each 



called the condensed-tissue stage, the mesenchymal period, and restricted the 

 term Vorknorpel to the earlier stages of the formation of cartilage. The 

 term " blastemal " is now-a-days commonly used to designate a mass of 

 mesenchymal tissue from which organs are to be differentiated, and is applied 

 to the tissue of the limb-bud before differentiation has commenced. It seems 

 to me that it would be well to extend this term to the structures first differ- 

 entiated in the limb. Thus, " scleroblastema " would mean the tissue differ- 

 entiated from the blastema of the leg and destined to give rise to skeletal 

 structures; myoblastema, the time differentiated for the muscles, and dermo- 

 blastema that destined for the skin. 



= The figures on this and the following plates are based upon embryos 

 belonging to the collection of Prof. Mall. I am greatly indebted to him for 

 the use of these embryos. 



