EEPEODUCTIYE AND UPJXARY ORGANS, 699 



as to take the form of an elongated duct known as the ductus vikUo- 

 infest inalis (see figs. 510 and 512). 



As the parts constituting the face are at first entirely absent, there 

 is necessarily no cavity corresponding to the mouth, but as the struc- 

 tures which give rise to the jaws and face come to be developed below 

 the cranium, tlie buccal and nasal cavities are generally deepened by the 

 increasing projection of these parts, and the mouth at last communi- 

 cates with the forepart of the primitive alimentary canal by an opening 

 formed into it at the fiiuces. The mouth, therefore, derives its lining 

 from the epiblast and forms no part of the original hypoblastic inflection 

 Avhich gives rise to the pharyngeal cavity. 



The posterior opening of the alimentary canal is formed at a con- 

 siderably later period than that of the fauces. When first produced by 

 the solution of continuity in the posterior reflection of the blastodermic 

 layers, it represents in mammals as well as in birds a cloaca, or part of 

 the primitive intestine common to the alimentary canal and the genito- 

 urinary passages. 



Reproductive and urinary organs. — .As completing the present 

 short notice of the development of the rudiments of the principal 

 organs of the embryo, there may, lastly, be mentioned the temporary 

 organs named the Wolffian todies, with their ducts and associated 

 parts, which are the precursors of, and are very constantly and 

 intimately associated with, the first origin and subsequent ca^oIu- 

 tion of the reproductive and urinary organs. These bodies, when fully 

 formed, constitute a pair of symmetrical organs which occupy nearly 

 the whole extent of the abdominal cavity, and consist mainly of shor't 

 transverse tubes presenting a glomerular vasctilar structure very similar 

 to that which exists in the glandular structure of the permanent 

 kidneys. They have thus been named the iJrimordkd kidneys (see 

 fig. 513, W, p. 702). 



The Wolffian bodies arise mainly in connection with the central 

 portion of the mesoblast, and as the permanent kidneys and their ducts, 

 the testicles and ovaries, and the respective male and female passages 

 are in their origin all intimately connected with the Wolffian bodies, 

 we may look upon the urinary organs and the internal reproductive 

 organs as equally products of the middle layer. The external sexual 



Fig. 511. — Human Ejibryo op about 



FOUR WEEKS (froiu KoUiker after A. 



Thomson), f 



/, the anterior limb rising as a .semi- 

 circular plate from the lateral ridge. 

 (The tigure is elsewhere desciibed). 



organs are integumental in their 

 origin, and may be considered 

 as aridng in the epiblast and 

 mesoblast jointly. 



The Limbs. — The limbs do 

 not commence till after the rudi- 

 ments of all the organs already 



referred to have made their appearance. They are to be regarded as 

 out-growths from the lateral part of the trunk, and take their origin 

 by a sort of budding out or extension of the elements composing the 



