204 THE MICROSCOPIST. 



a temporary appendage, the foetus has other special struc- 

 tures, derived from the blastoderm. Thus the somato- 

 pleure grows up over the embryo and forms a sac filled 

 with clear fluid, the amnion. The outer layer of the sac 

 coalesces with the vitelline membrane to form the chorion. 

 The atlantois begins as an outgrowth from the mesoblast. 

 It becomes a vesicle, and receives the ducts of the primor- 

 dial kidneys or Wolffian bodies, and is supplied with blood 

 from the two hypogastric arteries which spring from the 

 aorta. The allantois is afterwards cast off by the contrac- 

 tion of its pedicle, but a part of its root is usually re- 

 tained, and becomes the permanent urinary bladder. In 

 the Mammalia the allantois conveys the embryonic ves- 

 sels to the internal surface of the chorion, whence they 

 draw supplies from the vascular lining of the uterus. 



Foster and Balfour recommend that the study of em- 

 bryonic development should commence with the egg of a 

 fowl taken at different times from a brooding hen, or an 

 artificial incubator. The egg should be placed on a hol- 

 low mould of lead in a basin, and covered with a warm 

 solution of salt (7.5 per cent.). It should be opened with 

 a blow, or by filing the shell. With the naked eye or 

 simple lens, lying across the long axis of the egg, may be 

 seen the pellucid area, in which the embryo appears as a 

 white streak. The mottled vascular area, with the blood- 

 vessels, and the opaque area spreading over the yelk, may 

 be observed. The blastoderm may be cut out with a sharp 

 pair of fine scissors, floated into a watch-glass, freed from 

 vitelline membrane and yelk, and removed (under the salt 

 solution) to a glass slide. A thin ring of putty may then 

 be placed round the blastoderm, which is covered with 

 salt solution, and the thin glass cover put on. With a 

 low-power objective many of the details of structure may 

 be seen in an embryo of thirty-six to forty-eight hours 

 incubation, as the heart, the neural tube, the first cere- 



