703 THE FCETAL MEMBRANES. 



It is also worthy of notice that from the time Avhen the allantois has 

 attained some size, it, Hke the amnion, is possessed of contractility, 

 ■which probably resides in its external layer ; and accordingly, on open- 

 ing an incubated egg, from the effect of change of temperature or 

 other stimuli, active motions may be perceived, caused by the alternate 

 contraction and relaxation of different parts. 



In mammalia the origin and early development of the allantois are 

 nearly the same as in birds, but in a more advanced stage of develop- 

 ment, the important connection which the outer layer of this membrane 

 has with the formation of the vascular part of the chorion and foetal 

 placenta, modifies considerably the relations of the membrane to the 

 other parts of the ovum. In all of them, however, the two layers of the 

 allantois (splanchnopleure and hypoblast) are easily distinguished from 

 each other, the internal being entirely devoid of blood-vessels, of a 

 simple cellular structure, and containing the fluid with which the inner 

 sac of the allantois is filled. The external layer, on the other hand, is 

 highly vascular, and is composed of fibro-cellular and contractile fibrous 

 elements. 



In the ruminants, pachydermata and the cetacea, the allantois attains to very 

 large dimensions, extending widely into the greatly elongated ovum. In the 

 carnivora it passes round the middle of the ovum externally in accordance with 

 the zonal form of their jjlacenta. while in the rodentia and in man its vesicular 

 or deeper membrane at least, containing the fluid, has a much more limited 

 expansion, and stops apparently in its growth as soon as it has assumed the flask- 

 like form and has reached the interior of the chorion. This appears to be the 

 most probable explanation of the appearance, described by several embryologists, 

 and observed also more than once by the writer, of a pyriform space extending 

 in early human ova from the umbilicus to the inside of the chorion at the place 

 where the ]ilacenta is beginning to appear or ■ndll afterwards be formed. (.See 

 a recently described case by W. Krause in Reichert and Dubois, Ai-chiv, 187;").) 

 But in this and all other fonns the umbilical vessels which pass out of the 

 embryo are i:)laced externally to the vesicle of the allantois or its continuation 

 by the ui-achus towards the iirinary bladder : and these vessels undergoing an 

 extremely rapid development, pass off into the chorion and placenta, which 

 thus owe their- vascular structures to the outer layer of the allantois. 



In the human subject the allantois is both of very early formation, and its 

 non-vascular or internal part ceases to extend itself at a very early period, that is. 

 before the end of the fouith week. But already by this time the blood-vessels of 

 the outer layer, by themselves or more probably in association with a connective- 

 tissue layer in which they were originally situated, have overrun the whole 

 interior of the chorion, and very soon furnish to the developing villi of that struc- 

 ture, the fibrous element with vessels, of which they secondarily become possessed. 

 The manner of the completion of this jirocess will be apparent from what 

 follows, as to the formation of the chorion (Von Baer, Reichei-t, Remak, 

 Ivolliker). 



The Chorion. — The ovum of the mammifer when it enters the cavity 

 of the uterus is covered only by the vitelline membrane, or zona pellu- 

 cida, which is of ovarian origin, and as a rule (notwithstanding the 

 apparent exception of the rabbit to be afterwards referred to) it does 

 not appear that it acquires any other covering for some days after its 

 arrival in the uterus. By the time, however, that it becomes fixed in 

 that part of the uterus which it is to occupy during the subsequent 

 period of its intrauterine life, a great change takes place in the nature 

 of the external covering of the ovum, by its conversion into a new 



