202 



SCIENCE. 



[N. S. Vol. XIV. No. 345. 



forms between the fishes and the stego- 

 cephali. 



The thoracic duct of the mammals is the 

 primitive lymphatic duct of the left side ; 

 that of the right is greatly reduced and re- 

 ceives no lymph from the lower part of the 

 body. Exactly the same conditions occur 

 in the urodeles, but not in the reptiles. 



Another item of interest in this connec- 

 tion is that mammals and amphibia get rid 

 of their nitrogenous waste in the form of 

 urea, while reptiles void uric acid. 



As we saw earlier, embryology was an 

 important factor in directing attention to 

 the reptiles as ancestors of the mammals, 

 but now the weight of its evidence is in 

 the opposite direction. The fact that the 

 monotreme eggs are meroblastic is far from 

 conclusive, since similar conditions have 

 arisen independently several times in the 

 animal kingdom. Hubrecht, however, has 

 pointed out that certain other features of 

 development — those connected with the 

 foetal envelopes of the mammals — are not 

 to be derived from the conditions known in 

 any reptile, but that they are easily ex- 

 plained as arising from a type of egg found 

 in the amphibia. As these arguments, so 

 far as I am aware, have not been summar- 

 ized in English, they may be given in a 

 brief form here, omitting all points which 

 have no immediate bearing upon the ques- 

 tion at issue, such as the two types of fcetal 

 circulation, the nutritive functions of the 

 trophoblast and the like. 



As is well known, the mammals, like the 

 sauropsida, form a foetal envelope, contin- 

 uous with the sides of the body — known as 

 the amnion ; and from the fact that the 

 sauropsida are lower than the mammals, 

 the natural view has been that the reptilian 

 type has been the ancestral one, from which 

 that of the mammals has been derived. 

 Were this envelope to arise in all mammals 

 in the same way that it does, for instance, 

 in the sheep or the rabbit, this conclusion 



could not be gainsaid ; but when the am- 

 nion of the guinea-pig, the hedgehog, the 

 flying fox and that of man are considered, 

 we meet conditions which it is extremely 

 difficult, if not impossible, to explain in 

 such a way. 



Forget for the moment the well-known 

 diagrams of amnion formation which ap- 

 pear in any text-book, for they will confuse. 

 In the hedgehog there arises very early in 

 development a two-layered vesicle, the 

 layers being, according to Hubrecht's in- 

 terpretation, ectoderm and entoderm. At 

 one end of the vesicle the ectoderm is much 

 thicker than elsewhere, and projects like a 

 cone or papilla into the central cavity. 

 Soon a splitting occurs in tliis ectodermic 

 thickening, so that the whole structure now 

 forms a double vesicle, its two cavities being 

 separated by a partition formed of ectoderm 

 and entoderm, the larger and older cavity 

 having walls of ectoderm and entoderm, 

 the later one walls of ectoderm alone. 

 From this partition the embryo will arise. 



There are now clearly two kinds of ecto- 

 derm present in the germ ; one the embry- 

 onic, the other forming the outer walls of 

 both vesicles. For this latter Hubrecht has 

 proposed the name trophoblast in allusion to 

 its nutritive functions, and he distinguishes 

 two kinds or regions of trophoblast ; that of 

 the smaller cavity being called the allan- 

 toidan, that of the larger the omphalidian 

 trophoblast, from their future relations to 

 allantoic and vitelline circulations. 



With the development of the mesoderm, 

 which, of course, arises between the ecto- 

 derm and entoderm of the germinal area, 

 an important change is introduced. The 

 somatic sheet of this layer grows outward 

 and then turns upward into the angle be- 

 tween the embryonic ectoderm and the 

 omphalidian trophoblast, and then bends 

 downward on the inner side of the latter, 

 while the splanchnic mesothelium follows 

 the deeper surface of the entoderm into the 



