INTRODUCTION 
Very early stages of the human ovum are necessarily extremely rare. It 
is only by fortunate and fortuitous circumstances that an occasional speci¬ 
men comes into the hands of the investigator. Within recent years 
a number of young ova have been described, which have considerably 
extended our knowledge, and have served to show that in certain respects 
the early stages of development in man differ materially from those in 
lower mammals. The ovum of Hubert Peters, of which an account was 
published in 1899, still represents the youngest phase known. A specimen 
described by Leopold in 1906 is certainly earlier than that of Peters, 
but no embryonic rudiment was present, and in several other respects 
it must be considered abnormal. On the other hand, the ova described 
by Graf v. Spee (1905), Beneke, and Jung amply confirm, though they 
do not extend, the data provided by Peters’ specimen. 
Considerable light has been thrown on the problems involved in early 
human development by recent comparative work, more especially that of 
Selenka and Keibel on monkeys and apes, and of Hubrecht on Tarsius 
spectrum. It is now known that the Primates, including Tarsius in that 
category, form embryologically a group by themselves. All have certain 
common and peculiar features. There is always present a mesodermic 
connecting-stalk (Haftstiel), through which the vessels of the embryo and 
chorion are connected without the medium of an allantois; the yolk sac 
is very minute and is not coextensive with the blastocyst; there is a 
precocious extra-embryonic coelom lined by middle-layer cells, which are 
present at a very early period before the appearance of the primitive 
streak or embryonic axis, and therefore before the formation of the dorsal 
mesoderm of the embryonic body. 
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