GENERAL EMBRYOLOGY 1621 



the contiguous lateral plate. Eventually, however, the lateral 

 plate becomes detached. The intermediate ceU-mass on either 

 side gives rise to the internal genital and the renal organs. 



Each mesodermlc somite consists of cells, which are arranged in 

 a laminar manner around a central cavity or myoccele, which is of 

 temporary duration. These cells become disposed in three groups, 

 two being internal, and one external. Of the two inner groups the 

 mesial one, or that nearest to the neural canal and notochord, by 

 its mesenchvTne, constitutes a sclerotome, and the lateral group, or 

 that nearest to the temporary ca\'ity of the somite, forms a muscle- 

 plate. The external group of cells is generally regarded as giving 

 rise to the cutis- plate. 



The successive sclerotomes on either side proliferate and surroimd 

 the notochord and superjacent neural canal, forming for them a 

 selerotogenous or skeletogenous sheath of mesenchyme. This invest- 

 ment, as regards the notochord, represents the membranous verte- 

 bral column, and from it are developed (i) the vertebrae and their 

 ligaments, and (2) the inter\'ertebral discs, except the central pulp 

 of each, this piilp being a persistent part of the notochord. As 

 regards the neural canal, the selerotogenous investment gives rise 

 to the meninges of the brain and spinal cord. 



The successive muscle-plates give rise to myotomes (muscle- 

 segments) , and from these the striated muscles of the trunk, at least, 

 are developed. According to some authorities, they also give rise 

 to the muscles of the limbs; but another \'iew is that the limb- 

 muscles are developed from the core of mesenchyme in the centre of 

 each limb-bud, which core gives rise to the skeletal framework of 

 the limb. 



The cutis-plate, formed by the external group of somitic cells, 

 gives rise to the cutis vera or corium, the epidermis being of ecto- 

 dermic origin. According to some authorities, however, this group 

 of cells is concerned in the formation of muscular tissue. 



Mesodermic Somites ol the Head. — The first or most anterior primitive or 

 trunk mesodermic somite, derived as stated from the division of the column 

 ■of paraxial mesoderm, occupies the future occipital region. In front of this 

 region several head-somites or segments appear in lower vertebrates. They 

 are regarded as being nine in number, but they are less numerous in the 

 human embrj-o, there being probably three. They are formed bj' a segmen- 

 tation of the mesoderm of the head, as distinguished from the paraxial meso- 

 derm. 



Later Stages in the Fonn?.tion of the Embryo. 



The early stages in the formation of the embr\'o, just described, 

 are threefold, as follows: (i) The formation of the neural canal from 

 ectoderm, this canal being the rudiment of the entire cerebro-spina! 

 ner\'ous system ; (2) the formation of the notochord or chorda dor- 

 salis from entoderm, this chord constituting the axis roimd which 

 the vertebral column and its ligaments are developed from the 

 selerotogenous or skeletogenous sheath of the notochord, the chord 



