DEVELOPMENT OF LTSSENCEPHALA. 727 



vessels, and partially divide its cavity. The chorion receives 

 vessels from the vitellicle as well as from the aliantois. Gestation 

 in both Hare and Rabbit is 30 or 31 days: but the new-born 

 Hare is more advanced, the eyes being open. The Rabbit is 

 born blind. 



In the Guinea pig ( Cavia Cobaya, L.) the uterine ovum, when 

 the subdivisions due to cleavage-process have coalesced into the 

 germ-mass, has lost the hyalinion, and is surrounded by the cells, 

 nuclei, molecules, so-called ( epithelium,' c., formified from the 

 fluid of the amorphous lining matter of the womb, from which 

 the impregnated germ-mass is scarcely, if at all, distinguishable 

 (5th or 6th day): fluid accumulates in the centre of the germ- 

 mass, which IIOW T assumes the form of a cylinder with obtuse ends, 

 and may be said to be lodged in an ' utricular canal ' of the 

 decidua. When this substance has filled the part of the uterine 

 horn containing the cylindroid ovum, the end of this, next the 

 mesometral side, begins to receive vessels from the decidua, and 

 to be attached to such commencement of the maternal placenta : 

 the opposite or free end of the ovum is the seat of the initial steps 

 in the formation of the embryo. 



The cylinder expands into a sphere (12th day), the remaining 

 germ-mass forming the wall of which has become differentiated 

 into a serous or ( animal ' layer toward the centre or cavity of 

 the ovum, and into a peripheral, e vegetal' layer; 1 both expand- 

 ing to form the tunic of the ovum, everywhere in contact with 

 the thick surrounding bed of decidua. The fundamental structures 

 of the embryo rise from the part of the serous laver next the free 



/ / 



side of the uterine horn, and project into the cavity of the ovum, 

 toward which the dorsum of the embryo is turned. A vascular 

 layer is developed between the serous and the inner side of the 

 vegetal layers, the normal relations of the primitive germinal 

 strata being thus reversed. Bloodvessels extend from the widely 

 open abdomen or ventral surface of the embryo upon the vascular 

 layer of the ovum ; and, as the growing abdominal walls contract 

 to an ' umbilicus,' the embryo sinks into, or enters, the cavity of 

 the ovum, with which it is in communication by vitelline vessels, 

 fig. 573, b, defining a vitellicle by the ' vena terminalis,' ib. c, 

 (14th day). Very early a knob of nucleate cells, w r hich seem to 

 form the caudal end of the embryo, are developed into an aliantois, 

 which conveys allantoic vessels to the attached mesometral side 

 of the ovum, to ramify in the maternal placenta, ib. /, there formed 



1 The microscopic characters of these two layers are given in figures 36 and 37, 

 tab. Hi. CCLXII", 



