30 HUMAN ANATOMY. 



the latter becomes separated from the remaining portion of the somite and is probably 

 identified with the formation of the segmented excretory apparatus of the embryo, 

 the Wolfhan body, and hence is known as the ncphrotome.. 



The principal mass, includmg the greater part of the somite proper, consists of 

 an outer or peripheral zone of condensed mesoblast enclosing a core of looser struc- 

 ture. The less dense mesoblastic tissue later breaks through the surrounding zone 

 on the side directed towards the notochord and forms a fan-shaped mass of embryonic 

 connective tissue which envelops the chorda and grows around the neural canal. 

 The cell-mass derived from the core of the myotome constitutes the sclerotome, and' 

 directly contributes the tissue from which the permanent vertebrae and the associated 

 ligamentous and cartilaginous structures arise. The remaining denser part, the 

 myotome, which collectively forms a compressed C-Iike mass, becomes differentiated 

 into a lateral and a mesial stratum (Fig. 35). The lateral stratum, sometimes called 

 the cidis-plate , consists of several layers of closely packed elements. By some these 

 cells are regarded as concerned in producing the connective tissue portion of the 

 skin ; according to others they are in large part converted into myoblasts, which, with 

 those of the mesial stratum, or 7miscle-plate, give rise to the voluntary muscles of the 

 trunk. The genetic relations of the somite, therefore, may be expressed as follows : 



I Myotome — muscle segment. 



Somite -j Sclerotome— a.rza/ segment. 



(. Nephrotome — excretory gland segment. 



The number of somites of the human embryo is about thirty-seven, comprising eight 

 cervical, twelve thoracic, five lumbar, five sacral, and from five to seven caudal 

 segments. 



THE FCETAL MEMBRANES. 



The Amnion. — With the exception of fishes and amphibians, — animals whose 

 development takes place in water, — the young vertebrate embryo is early en\'eloped 

 in a protecting membrane, the amnion. Animals possessing this structure, including 

 reptiles, birds, and mammals, are classed, therefore, as amniota, in contrast to the 

 anainnia, in which no such envelope is formed. An additional foetal appendage, the 



alla7itois, is always de\'eloped 

 ^^*^- 36. as a structure complemental 



EmbYoiiic area ^Embr>'onic ectobiast to the amnion ; hence the am- 



»Mesobiast niota possess both amnion and 



iEntobiast allantois. 



Since the development of 

 the foetal membranes in man 

 presents certain deviations 

 from the process as seen in 

 other mammals, due to pecu- 

 liarities affecting the early 

 human embryo, it is desirable 

 ^ .,.,,. , . to examine briefly the forma- 



Cavity of blastodermic . , ^ 



Entdbiast vesicle tion of these Structures as ob- 



'Trophobiast Served in animals less highly 



specialized. 

 ^^^ Referring to the early 



Diagram of mammalian blastodermic vesicle. mammalian embryo, in which 



the blastodermic layers are 

 arranged as somatopleura and splanchnopleura on either side of the embryonic axis 

 and the surrounding uncleft mesoderm, and extend as parallel sheets over the en- 

 larging blastodermic vesicle, the first trace of the amnion appears as a duplicature 

 of the somatopleura. The earliest indication of the process is seen slightly in front 

 of the cephalic end of the embryo, the resulting head-fold being, however, soon fol- 

 lowed by the appearance of the lateral and tail-folds. The rapid growth of these 



