2 
INTRODUCTION 
There are other features, however, in which the several orders of 
the Primates differ inter se. In Tarsius the amnion is formed as in the 
rabbit, dog, etc., by secondary folds, while in monkeys, apes, and man it 
is already closed in the earliest stages known. The placentation again, in 
the monkeys (Old and New World), differs from that in the anthropoid 
apes and man. While the early phases in apes and monkeys, described 
by Selenka, confirm and explain the corresponding phases in the human 
subject, none of the stages known reach to the initial stages of the blasto¬ 
cyst, and therefore much is still left for conjecture. The extremely 
young ovum, which is the subject of the first of the pajoers in this 
memoir, represents the earliest stage of any primate form except Tarsius 
yet recorded, and merits careful and detailed description in respect that it 
pushes back the limits of the unknown in a sensible degree. 
The age of young human ova is, of course, from the nature of the 
case, quite uncertain. It is usually calculated in terms of the conventional 
rule formulated by Professor His, but the results of the rule as applied 
to the youngest known specimens are unsatisfactory and contradictory. In 
the present case we are fortunate in possessing very accurate data, and 
an effort will be made by correlating the facts with those known for other 
specimens to revise the basis on which the age of early ova is calculated. 
Not only do the structural features of the early primate blastocyst 
remain unknown, but the process of imbedding and the initial phases of 
placentation are also merely matters of surmise. All the ova described 
before the appearance of Hubert Peters’ monograph were found completely 
imbedded in decidua, and the hypothesis that the ovum becomes surrounded 
by a process of circumvallation was generally accepted, though in more 
recent times the results yielded by comparative embryology had caused 
some doubt on the matter in the minds of a few observers . 1 Several of 
1 In William Hunter’s Anatomy of the Gravid Uterus , 1774, Plate 35, there is figured a com¬ 
plete decidual cast, in which an ovum about the size of a pea lies imbedded. In his diagrams 
William Hunter clearly indicated that the ovum is at this stage completely surrounded by the 
decidua, but he expressed no opinion as to how it becomes implanted therein. The theory that 
the decidua covers the orifices of the Fallopian tubes, and is pushed out by the ovum as it 
enters the uterus, has been erroneously attributed to him. (See Historical Introduction to the 
Catalogue of the Anatomical and Pathological Preparations of Dr. William Hunter. John H. 
Teacher. MacLeliose, Glasgow, 1900.) 
