CHAPTER XLII. 



THE MESODERMAL SOMITES 



All the remaining systems to be described are intimately related to 

 the mesoderm. 



The mesodermal region, in the chick, posterior to the head, divides 

 into block-like segments by the formation of connective tissue septa 

 which form at right angles to the long axis of the embryo. So, too, in 

 the frog embryo and tadpole, the mesoderm divides into block-like seg- 

 ments. These are the somites. And, just as with the chick, so with 

 the frog embryo, the mesoderm divides into an outer somatopleure and 

 an inner splanchnopleure layer with an open space between them known 

 as the myocoele. 



That portion of the somites which lies directly on each side of the 

 notochord is known as the segmental or vertebral plate, while the por- 

 tion extending laterad from this segmental plate is called the lateral 

 plate. 



The lateral plates are therefore merely direct continuations of the 

 segmental plates. However, as the somites divide off into blocks, the 

 vertebral plates become thickened and not only is the myocoele closed 

 which lies within them, but a constriction along the long axis of the 

 embryo separates the vertebral plate from the lateral plate. 



As there are no somites in the head region, the mesoderm lies in 

 the form of scattered groups of mesenchymal cells in the head and 

 pharyngeal regions. 



The somatopleure and splanchnopleure are not of equal thickness. 

 The outer somatopleure is only one cell in thickness, while the inner 

 splanchnopleure is much thicker. ' Therefore, the coelom which lies be- 

 tween these two layers, lies much closer to the outer portion of the em- 

 bryo than to the inner portion. 



The one cell-layer which forms the outer somatopleure in the region 

 where the somites have formed is naturally segmented, as that layer is 

 an actual portion of the somite proper. These one cell-layered segments 

 of somatopleure are now called dermatomes or cutis plates, because they 

 will soon join with the ectoderm lying immediately above them to form 

 the outer wall of the embryo. 



The segments of the splanchnopleure which lie toward the center 

 of the embryo are called myotomes, or muscle plates, because it is from 

 these that the muscle cells will form. In fact, the formation of the mus- 

 cle fibrils can already be observed when the embryo is scarcely five milli- 

 meters in length. 



It is the thickening of the myotomes which obliterates the myocoele 

 quite early. 



