[128] 



OVUM. 



means of true ova, as well as by attached 

 buds, which last are then covered by a 

 strong corneous envelope, and have received 



Fig. 93*. 



Formation of buds in Paludicella. (From Van 

 Beneden and Dumortier.~) 



a. One of the Polypes of Paludicella Ehrenbergii 

 contracted within its cell, showing at the upper 

 part towards the right the commencement of the 

 formation of the bud by the growth of cells be- 

 tween the outer and inner layers of the cell-wall. 



b. The same bud a little more advanced and 

 more highly magnified, represented by itself. The 

 vesicular cells which separate the ectocyst and en- 

 docyst are seen more distinctly. 



c. A more advanced stage of the same, internally ; 

 the part from which the embryo polyped arises is 

 seen bulging out from the rest. 



This figure has been introduced to show the 

 difference between the process by which a true bud 

 arises and that by which ova are produced. 



the name of propagula. In Fredericella 

 they describe a propagation by means of 

 buds and by ova provided witli the strong 

 horny envelope. In Alcyonella and Lopho- 

 pus, besides the usual propagation by buds, 

 and by the common ova, these authors have 

 stated that there is also a viviparous produc- 

 tion of ciliated embryos from ova which re- 

 main within the parent animals ; but they 

 have not stated particularly the manner in 

 which these ova originate, nor their difference 

 from those which receive the corneous en- 

 velope. The difficulties presented by these 

 varieties seem to be in some measure re- 

 moved by the view offered by Professor All- 

 man of the nature of the bodies last men- 

 tioned, to which I will now advert. 



It has long been known that the so-called 



winter ova, or the bodies provided with the 

 corneous envelope, are formed chiefly towards 

 the autumn and winter sei'son ; and the 

 strength of their covering lias gene-'all v been re- 

 garded as a provision for the protection of the 

 germ from the hurtful influences of the winter 

 season. During iwo seasons I have observed 

 the production of these bodies from the Plu- 

 matella repens ; ana I have kepi ill em through 

 the winter till the polypes were developed, 

 and issued from them in the ensuing summer. 

 From his careful observation of these bo- 



Fig. 94*. 



Winter ovum and embryo of Lophopus Crystallinus. 

 (From Van Beneden and Dumortier.) 



This is the same as that represented by Turpin 

 under the name of " Cristatella mucedo." In A. the 

 flat surface, and in B, the narrow edge of the ovum, 

 is represented. The two valves of the egg cover- 

 ing have opened superiorly, and the embryo, which 

 already possesses three crowns of tentacles, is seeu 

 escaping. 



dies in several genera, Professor Allman has 

 arrived at the conclusion that they are not, as 

 was previously supposed, true ova, but rather 

 separated gemmules ; and he conceives that 

 Van Beneden, who has described their form 

 and structure so well, must have confounded 

 them with some other bodies in their first or 

 earlier stages, or has failed to distinguish be- 



O ' t O 



tween them and the true ova. This distinc- 

 tion Allman has succeeded in making by as- 

 certaining that the true ova and these bodies 

 do not arise in the same situation, and that 

 these winter ova or gemmules do not in their 

 earliest stages present any germinal vesicle or 

 macula as the true ova do, and do not after- 

 wards undergo any segmentation. They are 

 formed, according to Allman, in the funiculus 

 which connects the bottom of the stomach 

 with the inside of the cell of the polypide, the 

 same body which was described by Van Be- 

 neden and Dumortier as an ovary, but which 

 Allman regards rather as analogous to the 

 gemmiferous stolon of the solitary Salpae. 

 These bodies Professor Allman proposes to 

 call stato-blasts. He farther discovered that 

 there is a true ovary with genuine ova which 

 may be distinctly observed in Alcyonella, and 

 which is situated in the walls of the endocyst 



