INTRODUCTION. 



II 



individualized with respect to form and complexity 

 of structure. Here, accordingly, we meet with a 

 course of development analogous to that observed 

 in the limbs: reduction of the number of the con- 

 stituent parts, and specialization of the functions 

 belonging to these parts. With reference to this 

 we must not forget that the oldest mammalian 

 dentitions known to us possess all sorts of teeth in 

 exceptionally great number and with closed roots, 

 and that accordingly every incomplete dentition 

 must be the result of a process of development, 

 and that development frequently retrograde, or 

 from a higher to a lower type. 



Reproduction. — We may pass over the other 

 features in the structure of the mammals in order 

 to dwell at somewhat greater length on the peculi- 

 arities pertaining to their reproduction. And here 

 what we have to concern ourselves with is not the 

 number of young ones produced, which varies re- 

 markably according as the struggle for existence 

 is more or less easy for an animal, but the pheno- 

 mena on which certain subdivisions of the Mam- 

 malia have been founded. 



All mammals bring forth the young alive, and 

 the young are suckled by the mother for a certain 

 period after birth. Yet there are considerable 

 differences in the relations subsisting between 

 mother and young before birth. 



In some, which are called Didelphia or aplacental 

 mammals, there is no intimate connection between 

 the ovum and the maternal organs in which the 

 earliest development of the embryo is accomplished. 

 The envelopes of the embryo, the amnion, and the 

 ailantois, are indeed formed, but the latter mem- 

 brane does not enter into connection with the walls 

 of the uterus. The ovum remains entirely free, and 

 the embryo is ushered into the world in a compara- 

 tively backward stage of development, although, 

 indeed, provided with all the essential organs. 

 The monotremes and marsupials, which are thus 

 reproduced without a placenta, exhibit at the same 

 time all the marks of a strikingly low position in 

 the scale of being. Their brain is scarcely more 

 highly developed than that of reptiles, and as 

 regards the structure of the axial skeleton, as well 

 as that of the teeth and limbs, we have already 

 seen that it agrees in many points with that of 

 the oldest mammals. 



Much more intimate is the connection which 

 subsists between the fruit of the body and the 

 mother in the great majority of mammals, which 

 are called Monodelphia or placental mammals. 



In them the blood-vessels of the embryo, which are 

 brought through the ailantois to the surface of 

 the ovum, form in combination with those of the 

 uterus a special organ known as the placenta. The 

 vessels belonging respectively to the ailantois and 

 the uterus do not run into one another, or, in 

 technical language, anastomose, but return in 

 loops on each side, and the gaseous and liquid 

 substances contained in the blood that circulates in 

 them are exchanged solely by the process of osmosis, 

 that is, by filtering through the wails of adjoining 

 vessels. The embryo has its blood purified, and is 

 fed by means of the placenta. The blood of the 

 mother conveys to it the requisite supply of oxygen 

 and the other substances necessary for its life and 

 growth, and at the same time removes the carbonic 

 acid and other refuse products of its vital action. 

 The formation of the placenta is thus a fact of the 

 highest importance, and one can easily understand 

 why the first place has been assigned to it among 

 the characters on which a natural division of the 

 Mammalia is based. 



May this also be the case now with respect to 

 the form of the placenta.' I take the liberty of 

 doubting this, in spite of the authorities who lend 

 their support to the maintenance of this view. 

 When a division, based on the form of the placenta, 

 was first proposed I gave in my adhesion to it, 

 but now I believe, in view of more recent discoveries 

 and in consequence of certain more profound 

 investigations, that we are entitled to assign to the 

 form of the placenta only the value of a subordinate 

 character. 



We can indeed trace the development of the 

 placenta step by step in our mammalian forms. 

 The ovum is from the first surrounded by an 

 external membrane, the chorion, and this gradually 

 becomes covered with cellular processes called 

 villi, into which the embryonic vessels penetrate. 

 These villi get inserted into simple pits or de- 

 pressions in the mucous membrane of the uterus, 

 known as crypts, from which they can easily be 

 detached. Therewith begins the formation of the 

 placenta proper, and in many animals the relations 

 between the mother and the fruit of the womb 

 are confined to these simple vascular villi. But 

 the latter become more complicated; they give 

 off branches, penetrate further into the mucous 

 membrane, get aggregated into certain spots, which, 

 however, are distributed over the whole surface of 

 the ovum, and in that way form so-called cotyledons, 

 each of which consists essentially of a bundle of 



