ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 617 



deeply hollowed, tlie connections between the ectoderm and the axial 

 plate seem to become more intimate along its base, and, at the same 

 time, the groove becomes divided into proper endoderm and meso- 

 derm. It is the multiplication of the elements of this mesodermic 

 plate that causes the greater distinctness of the groove of the primitive 

 line. The axial plate of birds ought to be considered as the homologue 

 of the anus of Euscoui in Batrachians; it is rudimentary, indeed, its 

 lips being fused in a kind of autero-posterior median raphe, and it is 

 at these lips that we have most actively multiplying the elements 

 which are destined to form the mesoderm. At the bottom of the 

 groove the mesoderm grows at the expense of a cellular mass which 

 is common to the mesoderm and ectoderm ; but this fact must not be 

 thought to prove the origin of the middle from the outer germinal 

 layer. 



In combining with his own the results of other workers, M. Duval 

 takes occasion to consider the work of his predecessors. 



Physiological Purpose of Turning the Incubating Hen's Egg.* 

 — The sitting fowl frequently turns her eggs during incubation, and 

 when this process is carried on artificially, mechanical means must be 

 adopted to effect the same purpose. 



M. C. Dareste finds that during the first week of artificial incuba- 

 tion, eggs which are turned develope in essentially the same manner as 

 those which are allowed to rest, but the monstrosities which have 

 already been formed in the latter soon take on an excessive develop- 

 ment, and in very few eggs which are allowed to remain unmoved 

 during the whole period of incubation does the body-cavity of tho 

 embryo become closed in. The cause of death in the unmoved eggs 

 is, according to Dareste, the union by growth of the allantois with 

 the egg-yolk, which latter is thus prevented from becoming finally 

 absorbed into the alimentary canal preliminary to the closure of tho 

 body-cavity. These adhesions of the allantois with tho vitelline 

 membrane lead to frequent rupture of the latter, whose contents are 

 thus largely lost to the embryo. Death of the chick in tho unturned 

 eggs usually occurs about the second week of incubation. When the 

 eggs are turned over it is probable that the position of the allantois 

 upon the yolk is shifted, and this daily movement prevents adhesion 

 between tlie two surfaces. 



Sixteen eggs were placed under the same conditions of artificial 

 incubation, but eight were allowed to remain unmoved, while tho 

 eight remaining were turned over twice a day. In the first set absorp- 

 tion of the yolk did not occur in any specimen, and all the embryos 

 died in the course of the second or third week. In tho second set, in 

 six eggs the yolk was absorbed in the normal manner ; in a seveutli, 

 opened on tho twenty-second day, tho chick was alive and hearty and 

 the yolk was being absorbed ; in the eighth egg the chick was dead 

 on the twentieth day, and adhesion between tho allantois and yolk had 

 prevented absorption of the latter. 



♦ ComplcH Reudua, c. (1885) in». 8i:j-l. Pee Aimr. NutuniliMt, xix. (IHH.'j) 

 pp. 610-20. 



