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STUDY OF YOUNG CHICK EMBRYOS. 



Embryo Chick with about Twenty=four Segments and Three Gill Clefts {about 



Forty-six Hours' Incubation). 



The following description will apply almost equally well to embryos with 

 from twenty-six to twenty-nine segments. 



Examination in Toto. — The specimen as a whole, as in the fresh state, has a 



M.6. 



bl.is. 



Seg.t. 



Fig. 152. — Embryo Chick with about Twenty-four Segments. Surface View from the Dorsal Side. 



A.c.v, Amnio-cardiac vesicle. Am.f, Posterior edge of the amniotic fold. A.o, Arw opaca. A.p, Area pel- 

 lucida. A.vi, Arteria vitellina (or omphalo-mesaraica). bl.is, Blood-island in the area pellucida. Ht, 

 Heart. M.b, Mid-brain. ^(/, Medullary canal or spinal cord. Op, Optic vesicle. 01, Otocyst. Pi.g, 

 Primitive groove. Seg, Primitive segment. Seg.z, Segmental zone. X ^5 diams. 



grayish tint when viewed by transmitted light. As soon as it is hardened the 

 opacity of all the tissues is greatly increased. In the center of the germinal area is 

 the very conspicuous area pellucida, which, is somewhat pear-shaped. The por- 

 tion around the anterior end of the embryo (Fig. 1^2), A.p, is very wide. In the 



