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). 



781. 



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, c, the central vesicles have disappeared, owing to the development of the vitellus ; the 

 ovum presents the appearance of an agglomeration of ova, or the mulberry-like aspect. D, further 

 developed ovum, with double external membrane and intervening transparent liquid. E, the yolk 

 elongated, partially divided in the middle ; the larger portion becoming the trunk, and the narrow portion 

 the tail, of the larva. F, the embryo at the full term of incubation : a, anterior process ; b, eye ; c, integu- 

 ment. G, the same, further advanced ; the caudal portion entirely absorbed, 'leaving the tegumentary 

 elongation empty ; a, anterior process. H, the same, further advanced ; superficial appendages still 

 present ; internal organs more distinct, i, embryo nearly perfect ; a, branchial orifice with tentacles ; 

 b, anal orifice; c, eye; d, cesophageal collar; e, (?) ; ff, ciliated circles, rudiments of respiratory organs; 

 g, <7, g, muscular bands. 



The former of these becomes more arid more 

 elongated, following the outline of, and fold- 

 ing itself around, the body, which is included 

 in the ovum, as in many reptiles. 



In Amaroucium, according to Milne-Ed- 

 wards' observations, the tail is formed by the 

 marginal portion of the yolk being condensed, 

 at the same time that the body is flattened, and 

 then separated from it throughout its length. 

 Van Beneden, however, remarks, that in the 

 simple Ascidians, as we have stated above, the 

 tail is rather an elongation of one part of the 

 yolk, and that, like the exterior organs of all 

 other animals, it is formed by extension and 

 not by division or separation. 



The body part of the embryo has an in- 

 ternal cavity, and is formed of *the yolk; it is 

 surrounded by the membrane, which becomes 

 the skin of the body and tail. The caudal 

 appendage is hollow, and communicates with 

 the central cavity ; it does not in Ascidice pre- 



sent the spiral or zigzag cavity observed in the 

 Botryllidce. 



There is soon formed within the embryo 

 another layer distinct from the exterior, and 

 which is destined to become the parietes of 

 the intestinal tube. There exist then two 

 layers, internal and external, from which all 

 the organs are derived. In the thickness of 

 the external layer, near the middle of the body, 

 and rather superiorly, occurs a black pigment- 

 cell, which Van Beneden regards as a rudi- 

 mentary eye. In Amaroucium, Milne-Edwards 

 has observed one or two blackish oculiform 

 points towards the posterior extremity of 

 the body. 



The chorion is now ruptured, and the larval 

 embryo appears with a pellucid membrane 

 covering its body and tail (fig. 781. F). This 

 membrane has been already mentioned as in- 

 vesting the ovum at a late stage of the first 

 period of its development, immediately after 



