404 RECORD OF CURRENT RESEARCHES RELATING TO 



trace of its existence is left on the ectoderm. This last now forms a 

 closed vesicle enclosing the remainder of the vitelline mass or endo- 

 derm, and the changes which it undergoes during development obtain 

 throughout its whole extent. The endoderm grows into a mass which is 

 thick at its centre and more delicate towards its edges ; the gastrodiscus, 

 the median portion of which forms the beginning of the embryonic spot, 

 is formed by part of the ectoderm, and the endoderm. The marginal cells 

 of the latter become isolated, while the central cells adhere to one an- 

 other and form several layers. At the beginning of the sixth day the 

 peripheral gastrodiscal cells and the deeper layers of the central spot 

 become flattened and form a continuous hypoblast ; while the cells of 

 the central spot, which are not thus moditied, go to form the meso- 

 blast. The hypoblast and mesoblast are, therefore, derived from the 

 primitive endoderm. At the end of the sixth day, the outermost layer, 

 or epiblast, alters its character ; ceasing to be pavement cells, the con- 

 stituent parts become prismatic or cylindroidal, and at this period the 

 embryonic spot becomes apparent. The flat cells of the ectoderm and 

 those of the hypoblast present a very marked reticular structure, and 

 the protoplasm of the former is always partly made up of bacilliform 

 rods, which have a most remarkable resemblance to Bacteria. The 

 embryonic spot (germinal disk) is at first circular, and then passes 

 through an oval form to a pyriform ; in the earlier stages the central 

 is always much clearer than the peripheral portion, and it was pro- 

 bably this circumstance which led Bischoff to assert the presence of 

 an area pellucida and an area opaca. The change in form is due to 

 the enlargement of the posterior edge. The embryonic area is, from 

 the first, composed of two distinct regions. The mesoblast primitively 

 extends through the whole of the germinal disk ; at the commence- 

 ment of the seventh day it is only found at the posterior extremity 

 and at the sides of the embryo ; when this disappears from the circular 

 area that region becomes " didermic." 



Production of Sex.* — Keferring to an article contributed by him- 

 self to a Venetian journal, in which he attributes the determination 

 of the sexes to the number of sj)ermatozQa which enter the ovum, 

 M. Canestrini considers that experiments are needed in order to 

 ascertain what amounts of sperm are necessary for the production of 

 each sex. M. Thury's theory, that the lateness or earliuess of the 

 period of fertilization is the determining cause, though not an exact 

 theory, is yet favourable to that of the author ; for the reason why 

 fertilization early in the period of heat of the female produces female 

 offspring appears to be the distance at which the ovum then is from 

 the source of the sperm, and the consequently small number of sper- 

 matozoa which penetrate so far up the Fallojjian tube as to reach it. 



In some cases the sex may be said to be determined by accident, 

 as when, of many sperm-cells produced, but few meet the ovum owing 

 to a combination of unnoticed causes, and so a female ofi'spring is 

 the result. But given that the circumstances generally are favour- 

 able in both parents, and that the fertilization takes place towards 



* 'Eull. Soc. Ven.-Trent. Sci. Nat.,' i. (1879) p. 18. 



