36 
AN EARLY HUMAN OVUM 
cases among the lower mammals in which there is a decidua capsularis 
provide a competent analogy. 
The hedgehog presents certain resemblances in respect of the gross 
characters of its placentation to the human subject; there is not only a 
decidua capsularis, but also a general primary, and a definitive discoidal 
placenta. For these reasons, among others, Hubrecht undertook the 
investigation of the development of the placenta, which has led the way 
towards a clearer understanding of the human placenta. 
The ovum of the hedgehog while still a small hollow blastocyst settles 
down at the bottom of a groove in the uterine mucosa. The epithelium 
around it disappears and at the same time a decidual swelling occurs in 
both lips of the groove, which thus enlarge and meet over the ovum. 
A certain amount of destruction of the connective tissue of the mucosa 
takes place, bleeding results, and a plug of blood clot is formed between the 
lips of the decidual swellings. The epithelium next disappears from the 
contracting lips, and they coalesce to complete the inclosure of the ovum. 
The ectoderm of the blastocyst proliferates into a relatively thick 
lamella, which Hubrecht named the “ trophoblast.” It surrounds the whole 
blastocyst, and the formative cell mass projects into the interior as a 
knob of cells. The trophoblast on the one hand rapidly establishes a union 
with the proliferating decidua, and on the other becomes hollowed out into 
cavities, into which maternal blood is shed from the capillaries which 
have been opened up by its destructive activity. At first the trophoblastic 
shell is complete and the greater part of it becomes penetrated by meso- 
blastic villi which are vascularised from the yolk sac forming a primitive 
vitelline placenta, but the definitive placenta is formed by villi which 
receive their blood-vessels from the allantois. 
At a certain stage therefore there is a diffuse placenta, while the 
definitive placenta is implanted upon the area of decidua adjacent to 
the uterine wall, i.e. decidua serotina or basalis. As this develops the 
vitelline placenta applied to the decidua refiexa gradually undergoes 
atrophy. The main difference between these arrangements and those that 
prevail in man is that the primitive placenta is supplied in the latter by 
vessels which develop in the Haftstiel , or connecting stalk of mesoderm, 
