ORIGIN OF THE MESODERM 



457 



all outer coverings of the body as well as the nervous system are de- 

 rived from the ectoderm ; why the lining of both digestive and respira- 

 tory organs comes from the entoderm ; and why the circulatory system 

 as well as the blood, lymph, muscle and connective tissue (except neu- 

 roglia) are derived from the mesoderm. 



The primitive streak, relatively, seems to become pushed further 

 and further tailward, but this is due to the greater growth in the cephalic 

 region of the embryo. (Compare Figures 262 and 264.) 



The entoderm spreads out as a very definite layer of cells, and 

 merges peripherally with the inner margin of the germ wall, even over- 

 lapping it slightly. The little cavity between the yolk and this ento- 

 dermal layer which has been called the gastrocoele will henceforth be 

 known as the archenteron or primitive gut (Fig. 265). The student is 

 not to look for a cavity in his sections, however, as the yolk in this 



region, by the very fact that it is separated 

 from the entoderm and forms the floor of 

 the primitive gut cavity, will not adhere to 

 the embryo when it is removed for section- 

 ing purposes. 



At eighteen hours of incubation the cell 

 boundaries of the germ wall cannot usually 

 be seen, though there are many nuclei and 

 yolk granules, the latter in various stages 

 of absorption. Because the nuclei of the 

 germ wall arise by division of the nuclei of 

 the cells lying at the margins of the expand- 

 ing blastoderm, it is assumed they are in- 

 strumental in breaking up the yolk in ad- 

 vance of the arrival of the spreading ento- 

 derm about the yolk sphere. 



At about twenty-two to twenty-three 



hours of incubation a pocket of entoderm can be seen in the anterior 

 region by examining the whole mount, and focusing through the ecto- 

 derm. This is the first formation of a gut floor in addition to the yolk 

 which has been answering that purpose up to this moment. This pocket 

 forms the fore-gut. 



The mesoderm grows laterad and then extends cephalad, so that 

 an area between the two cephalad growing portions of mesoderm is 

 formed, which area is called the proamnion (Fig. 266, P). It is merely 

 an open space and must not be thought of as forming the later true 

 amnion. It is to be noted primarily, because it permits a better study 

 of just how the mesoderm grows in relation to it. It will be well to ob- 

 serve the difference in this space in eighteen and twenty-three hour 

 embryos. 



As the mesoderm begins its growth where it does, there is none of 



Fig. 264. 



Surface view of a twenty-one 

 hour chick embryo, in which the 

 head-fold and first pair of primi- 

 tive mesodermal segments are pres- 

 ent. (After Duval.) 



