366 HISTOLOGY 



Since, however, he frequently referred to the entire follicles as ova, his results were not 

 promptly accepted; the diameter of the isthmus of the tube is so small that the entrance 

 of the follicles into the uterus was considered impossible. It was a matter of easy 

 observation to determine more precisely the relation of the ova to the follicles. After 

 many years this was done by Von Baer, an eminent embryologist, whose studies 

 of the chick are regarded as " the most profound, exhaustive and original contribution 

 to embryology which has ever been made" (Minot). This work bears the famous 

 subtitle "Beobachtung und Reflexion" the German expression of Haller's "Observa- 

 tions suivies de Reflexion" and De Graaf's " Cogitationes atque obseroationes" After 

 describing the condition of the ova in the tubes of the bitch, Von Baer writes: 



"It remained for me to ascertain the condition of ova in the ovary, for it seemed 

 clearer than day that ova so small as those found in the tubes did not represent Graafian 

 follicles expelled from the ovary; and I did not consider it probable that such solid bodies 

 had been coagulated from the fluid of the vesicles. Now, contemplating the ovaries 

 before making an incision, I clearly distinguished in almost every vesicle, a yellowish- 

 white point unattached to the walls, which swam about freely in the fluid when the 

 vesicle was pressed upon with a probe. Led on by a certain curiosity, rather than 

 moved by hope that with the naked eye I had seen ovules in the ovaries through all 

 the coats of the Graafian follicle, I opened a vesicle, and taking out a point in question 

 on the blade of a knife, I placed it under the microscope. I was overcome with amaze- 

 ment when I saw the ovule, now recognized outside of the tubes, so clearly that a blind 

 man could hardly doubt it. Surely it is strange and unexpected that an object so per- 

 sistently sought for, and endlessly described as inextricable, in every physiological 

 compendium, could so easily be placed before the eyes" (De ovi genesi, Lipsiae, 1827). 



Thus the ova in mammalian ovaries, which had long been believed to exist, were 

 first definitely seen within the follicles one hundred and fifty years after the discovery 

 of the microscopic spermatozoa, the existence of which had never been suspected. 



THE DECIDUAL MEMBRANES OF THE UTERUS AND EMBRYO. 

 DEVELOPMENT AND GENERAL FEATURES. 



Before describing the mucous membrane of the uterus during preg- 

 nancy, it is necessary to consider the membranes which envelop the 

 embryo. Although these are in contact with the lining of the uterus and 

 in part intimately blended with it, they are portions of the embryo itself. 

 The external membrane, toward the uterus, is known as the chorion; the 

 inner membrane, toward the embryo, is the amnion. Since the embryo 

 receives its nutriment from the wall of the uterus through blood vessels 

 in the chorion, these membranes develop very early and thus provide for 

 rapid growth. They are already present in the youngest human embryos 

 which have yet been obtained. 



Of the fertilization and segmentation of the human ovum, which 

 doubtless take place in the upper part of the uterine tube, nothing is 

 known except by inference from lower animals. The four-celled stage 

 has been observed once in a monkey, but the youngest known human 

 embryo is already provided with ectoderm, mesoderm and entoderm, and 

 has entered the uterus. As a purely hypothetical figure, we venture to 



