EGGS OF BIRDS. 



251 



129 



the oviduct, upon which they form a delicate muscular tunic, 

 whose fibres embrace the oviduct for the most part in the trans- 

 verse or circular direction, except at the infundibular aperture, 

 where they affect the longitudinal direction, which enables them to 

 dilate that orifice. Longitudinal muscular fibres begin again to 

 be distinctly seen in the uterine portion of the oviduct, whence 

 they are continued along the so-called vagina. An internal stratum 

 of circular fibres is also situated immediately behind the calcifying 

 membrane of the * uterus.' In the vas-ina the circular fibres are con- 

 centrated at its termination to form the sphincter above mentioned. 



The 'clitoris' of the Ostrich is continued from the anterior 

 margin of the preputial cavity of the cloaca, and is grooved like 

 the penis of the male : it is furnished mth corresponding muscles. 

 A smaller clitoris exists in those birds of which the males have a 

 well-developed intromittent organ. 



§ 169. Fecundation iri Birds, 

 and Structure of the laid Egg. 

 — In coitu spermatozoa enter 

 the cloaca and penetrate the 

 o\dduct, ascending to the ova- 

 rium. The germinal vesicle, 

 on the reception of the ovum 

 by the OAiduct, is no longer 

 visible, as such. A discoid 

 acroTeo^ate of cells constitutes 

 an opaque white circular spot 

 on the part of the periphery 

 of the yolk to which the germ- 

 cell and germ-yolk had passed, 

 and this was known to the 



. structure of the cicatricula in a laid Fowl's egg. cccviii 



older embryologists as the 



' cicatricula.' It consists of a central clearer and of a peripheral 

 denser portion, fig. 129, B : beneath the clear centre is a group 

 of minute opaque granules called ' nucleus cicatricula3.' In the 

 diagrammatic figure A, a is the vitelline membrane ; d the clear 

 tract leading from the ' nucleus,' c, to the centre of the yolk, 

 — the trace of the excentric course of the germ-cell : b, b, are the 

 minute granules forming the denser part of the cicatricula ; e, e, 

 are the larger yolk corpuscles. The ' nucleus cicatricula^,' c, is 

 the ' germ-mass,' the result of the same series of spontaneous 

 divisions of the impregnated germ-cell, as affected the entire yolk 

 in the Batrachian (vol. i. fig. 452) ; to which the ovum of the Bird 

 offers the opposite condition in the preponderance of the ' food- 



