PARABLAST THEOKY OF HIS. 25 



Before proceeding to describe the commencing development of the embryo it will 

 be instructive to enumerate the parts which are formed respectively from the three 

 blastodermic layers. The following is the relation given in tabular form : 



The whole of the nervous system, including not only the central organs (brain and 

 spinal cord), but also the peripheral nerves and sympathetic. 



The epithelial structures of the organs of special sense. 



The epidermis and its appendages, including the hair and nails. 



The epithelium of all the glands opening upon the surface of the skin, including the 

 mammary glands, the sweat glands, and the sebaceous glands. 



The muscular fibres of the sweat glands. 



The epithelium of the mouth (except that covering the tongue and the adjacent 

 posterior part of the floor of the mouth, which is derived from hypoblast), and that of 

 the glands opening into it. The enamel of the teeth. 



The epithelium of the nasal passages, of the adjacent upper part of the pharynx, 

 and of all the cavities and glands opening into the nasal passages. 



The urinary and generative organs (except the epithelium of the urinary bladder 

 Q 4^ and urethra). 



^ ji All the voluntary and involuntary muscles of the body (except the muscular fibres 

 g -g ( of the sweat glands). 



The whole of the vascular and lymphatic system, including the serous membranes 

 ^ ^ and spleen. 



The skeleton and all the connective tissue structures of the body. 



The epithelium of the alimentary canal from the back of the mouth to the anus, 

 and that of all the glands which open into this part of the alimentary tube. 

 The epithelium of the Eustachian tube and tympanum. 

 The epithelium of the bronchial tubes and air sacs of the lungs. 

 The epithelium lining the vesicles of the thyroid body. 

 The epithelial nests of the thymus. 

 The epithelium of the urinary bladder and urethra. 



PAEABLAST THEOBY OP HIS. MESENCHYME THEORY OP 



The observations of His upon the development of the blood and connective tissues 

 in the bird led him to regard these tissues as originating, not from the mesoblast which 

 in the chick grows out from the sides of the primitive groove, but from cells which, 



, 



M 



Fig. 27. VERTICAL SECTION THROUGH THE BLASTODERM OF A HEN'S EGG TAKEN NEAR THE 



PERIPHERY. (Strieker.) 



E, epiblast ; II, hypoblast, passing at the periphery into an undifferentiated mass of yolk, A, 

 containing large cells filled with yolk granules ; M (towards the centre of the blastoderm), mesoblast ; 

 M (nearer the periphery), granular cells, apparently derived from A, and lying between the epiblast 

 and hypoblast. 



originating either in the yolk or in the thickened rim of the spreading blastoderm, wander in 

 centripetally between the primary layers and fill up all the interstices of the centrifu gaily - 

 growing true mesoblast. These in-wandering cells being derived, not like the other cells of 

 the embryonic area from the more active primarily differentiated central parts of the blasto- 

 derm, but from the peripheral non-embryonic portion, were collectively named by His 

 jiariiblaxt, and the tissues (blood and blood-vessels, and all the connective tissues) supposed to 

 be formed from them were termed paraUastw (all the other tissues of the embryo being 

 termed, in contra-distinction, archiblastic'). 



His's theory was enunciated as long ago as 1868, although he afterwards introduced 

 into it certain modifications. For -a considerable time it met with little Acceptance, but of 



