THE DEVELOPMENT OP THE EMBRYO -ITSELF. 97 



using a hand lens, though this is 

 knowledge ; in fact, if the exam- 

 ination goes no further than the 

 naked-eye appearances of a dozen 

 eggs, selecting one eveiy twenty- 

 four hours during incuhation, 

 when opened and the shell and 

 membranes well cleared away, 

 such a knowledge will be sup- 

 plied as can be obtained from no 

 books or lectures however good. 

 It will be, of course, understood 

 that the student approaches these 

 examinations with some ideas 

 gained from plates and previous 

 reading. The latter will furnish 

 a sort of biological pabulum on 

 which he may feed till he can 

 supply for himself a more natu- 

 ral and therefore more healthful 

 one. While these remarks apply 

 with a certain degree of force to 

 all the departments of physiolo- 

 gy, they are of special impor- 

 tance to aid the constructive fac- 

 ulty in building up correct no- 

 tions of the successive rapid 

 transformations that occur in 

 the development of a bird or 

 mammal. 



rig. 99 shows the embryo of 

 the bird at a very early period, 

 when already, however, some of 

 the main outlines of structure 

 are marked out. Development 

 in the fowl is so rapid that a few 

 days suffice to outline all the 

 principal organs of the body. In 

 the mammal the process is slow- 

 er, but in the main takes place in 

 the same fashion. 



As the result of long and pa- 

 1 



not essential .to getting a fair 





Fm 



viF 



Pi: 



ZTh- 



Stz 



Fig. 99.— Embryo fowl 3 mm. long, of 

 about twenty-fonr hoars, seen from 

 above. 1 x 39. (Haddon, after 

 KOlliker.) Mn, nnion of the med- 

 ullary folds in the region of the 

 hind-brain; Pr, primitive streak; 

 Pz, parietal zone ; Bf, posterior 

 portion of widely open neural 

 groove; Rf, anterior part of neu- 

 ral groove ; Mw, neural ridge ; Stz, 

 trunk-zone ; vAf, anterior amni- 

 otic fold ; vD, anterior umbilical 

 sinus showing through the blasto- 

 derm. His divides the embryonic 

 rudiment into a central trunk-zone 

 and a pair of lateral or parietal 

 zones. 



