THE MESODERM. 23 



tion of any kind is visible. Ectoderm, mesoderm, and yolk cells form a common, 

 thickened rim, or germ wall, similar in general appearance to the early stages of 

 the primitive streak, and extending along the entire lateral margins of the germinal 

 aiea. (Figs. 140-142.) 



The post-oral mesoderm therefore arises from three distinct sources. The 

 axial portion, consisting of the double line of mesoblastic somites, arises from the 

 primitive streak; it represents the trail of mesoderm cells left behind as the telo- 

 blasts of the primitive streak migrate backward. The greater part of the lateral 

 plate mesoderm is formed from the proliferating cells of the germ wall, as it 

 spreads over the surface of the yolk in a lateral direction. But on the median 

 side of the germ wall, the definitive ectoderm continues to proliferate inward for 

 a considerable distance along the lines that separate the lateral plates. The cells 

 thus produced form a part of the lateral plates, and the proliferating lines break 

 the lateral sheet of mesoderm into distinct segments. 



The dicephalic and mesocephalic (thoracic) mesoderm of arachnids presents 

 a most important modification. It forms at first, a well defined band on either 

 side of the nerve cord. Each band then becomes divided into distinct ccelomic 

 chambers or somites; but segmented lateral plates are absent, the mesoderm of 

 that region consisting of scattered cells that are not visible in surface views. 

 (Figs. 15, 1 6, 19-21.) From the thoracic somites, or head cavities, arise the 

 muscles of the appendages, the cartilaginous cranium, and the secreting cells of 

 the coxal gland, or head kidney. 



The procephalic mesoderm is scanty and unsegmented, forming a thin 

 walled, unpaired coelomic vesicle that breaks down into scattered cells. The 

 procephalic mesoderm appears to arise from the primitive cumulus before apical 

 growth begins. 



Comparison. With the progress of cephalization in the arthropods, there 

 has been, therefore, a steady decrease in the volume of mesodermic structures. 

 In the higher arachnids, mesoderm is almost absent in the procephalon, and the 

 lateral plates are absent in the dicephalic and mesocephalic regions. The result, 

 or cause, if you will, is the absence of the thoracic sections of the heart and of the 

 longitudinal, intersegmental muscles; the shortened thoracic tergites then fuse 

 with one another and with the procephalon to form a continuous unsegmented 

 shield, or cephalic buckler. 



In the vertebrates, the decrease in volume of the cephalic mesoderm is carried 

 still further, affecting the anterior head regions, as well as the more posterior 

 ones, that in the arthropods are usually well equipped with mesodermic structures. 

 This decrease is due chiefly to the progressive atrophy, or fusion, or condensation 

 of what were originally freely movable parts, and the consequent reduction in the 

 number and volume of cranial muscles. For example, practically all the haemal, 

 longitudinal, intersegmental muscles disappear with the fusion of the branchial 

 region with the head. The several pairs of originally separate leg-jaws fuse into 

 unpaired oral arches, only one of which is freely movable. The mesocephalic 



