. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. 1212 TUNICATA. fluid layer, another thin transparent membrane, an organised layer forming the periphery of the vitellus, and lastly of the soft yolk in the centre, the vitelline membrane and the blas- toderm forming one with the vitelline mass. The external membrane and the albuminous layer beneath it appear to be formed exter- nally to the ovum by the oviduct or the ovary; and as they envelop the ovum after its com- plete development, they may be considered only as accessory parts. It being uncertain whether the ova in
. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. 1212 TUNICATA. fluid layer, another thin transparent membrane, an organised layer forming the periphery of the vitellus, and lastly of the soft yolk in the centre, the vitelline membrane and the blas- toderm forming one with the vitelline mass. The external membrane and the albuminous layer beneath it appear to be formed exter- nally to the ovum by the oviduct or the ovary; and as they envelop the ovum after its com- plete development, they may be considered only as accessory parts. It being uncertain whether the ova in which these changes take place, have been subjected to fecundation whilst still in the ovary or ovi- duct, there arises the interesting question as to what extent of modification, even to the disappearance of Purkinje's vesicle, and the mulberry-like condition of the vitellus, can take place before the ovum has received the stimulus of the seminal element. During the changes of the ovum in this first period of its development, the colour of the egg is occasionally subject to certain modifica- tions. In Ascidia grossularia, the ova become of a bright red colour, and in some species no change of colour takes place. In Amaroucium, a compound Ascidian, the ova change from a pale to a deep yellow. The second period of development is marked by the prolongation of one side of the yolk to form the caudal appendage. The ovum has now a chorion, albumen, and yolk, the two latter being separated by a fine membrane. The vitelline or embryonic mass becomes con- tracted about its middle, and is somewhat bean-shaped. From this time there are ob- servable two extremities, one lengthening it- self, as the other becomes more and more globular; and respectively representing the caudal appendage and the body (Jig. 781. E). f \ a. Development of Cynthia ampulla. (After Van Beneden.~) A, ovum, with the vesicles of Purkinje and Wagner. B, the same further advanced, with surrounding vitelline globules
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