Oct. 1, 1868.] 



HARDWICKE'S SCIENCE-GOSSIP. 



227 



bad swallowed the other ! Yet we believe the 

 phenomenon admits of a very simple explanation. 



It must be kept steadily in mind that only the 

 yolk-bag and contents are developed in the ovary, 

 and that the white (or albumen) and the shell are 

 afterwards added to complete the egg during the 

 transit of the yolk-bag along the oviduct. 



Now, if from any cause an egg is detained for an 

 unusual length of time in the oviduct, its walls are 

 re-excited to action— a second " white " (varying in 

 quantity in different instances) and a second shell 

 are secreted around the first egg, and thus a double 

 egg is formed; but the central egg only, in such 

 cases, would be fertile. Our own example, as it 

 contained two perfect yolks, will therefore require 

 a modification of the foregoing explanation fully to 

 clear up the nature of the process. 



We must suppose a yolk-bag to have left the 

 ovary of the blackbird, entered tbe oviduct, and 

 there received, as usual, its " white " and shell, but 

 was detained by spasm, or some other kind of ob- 

 struction, until a second yolk-bag entered the 

 oviduct ; this, urged on by the peristaltic action of 

 the tube, would at length impinge on the egg in 

 front. Now, its membrane being very extensible, 

 and the vis a tergo still continuing, the first egg 

 would after some little time be completely invested, 

 though of course not in the cavity of the yolk-bag, 

 just on the principle of a closed or double nightcap. 



This investiture being accomplished, there only 

 remains for the albumen and shell to be deposited 

 in order to complete the double egg. 



When two yolks ore found in one shell, the ex- 

 planation is as follows : — " Two yolk-bags become 

 adherent, and so enter the egg-duct together, where 

 they receive a common white and common shell." In 

 this case the egg is unusually large, and there is no 

 smaller end; so that a double yolk can be predicted 

 beforehand. 



Newcastle, Staffordshire. James Yates. 



THE HEART OE DAPHNIA. 



rPHE great distinctive character which separates 

 -*- the Vertebrata from the Articulata is, without 

 doubt, the attachment of the muscular system, in the 

 one sub-kingdom to au internal osseous framework 

 {endoskeleton), in the other to an external, jointed, 

 and chitinous case {exoskeleton) . 



Next in importance to this, is the position of the 

 circulatory and nervous systems. In the Vertebrata, 

 the situation of the heart and large blood-vessels is 

 ventral, and that of the neural system, dorsal. In 

 the Articulata, this is exactly reversed ; the heart 

 being dorsal, and the chain of nervous gangUa 

 ventral. Among the other distinctions between 

 these two natural divisions of animal life may be 

 remarked the comparatively incomplete or lacunar 



character of the circulation in the Articulata. Taking 

 the Insecta as a type of the series, we find a long 

 dorsal vessel or heart, lying bathed in the nutritive 

 fluid, which enters into it by more or less numerous 

 slits or openings opposite to each other, and which 

 divide the heart into sections or chambers, cor- 

 responding generally to the annulations of the ex- 

 ternal case. The successive muscular contractions 

 of these chambers propel the blood towards the 

 head of the animal, and it finds its way back again 

 through wide lacunar channels ; or is vaguely 

 filtered, as it were, through the various structures 



-^§£ 



B 



130. 



Fig-. 220. Diagram of the Heart of Daphnia x 

 A. Daphnia puUx. B. Daphnia vetula. 



C. Dorsal aspect of the Heart of D. pulex. 

 The arrows above the figures point towards the head of the 

 animal. The small arrows indicate the direction of the 

 currents of the blood. 



and organs of the body. In so vast a series as the 

 Articulata, of course great variety is to be found in 

 the completeness of the circulatory system ; and it 

 will be noticed that where there is a more local 

 respiratory arrangement, as in the case of the 

 higher Crustacea and the pulmonary Arachnida, the 

 vaseular provision is much more perfect ; while in 

 some of the lower forms of the sub-kingdom it is 

 partly or altogether absent. 



Several well-known and favourite objects will 

 display, under the microscope, various parts of this 

 circulation ; but a sight of the typical fenestrated 

 heart of an articulate animal, in full action, is not, I 

 think, very common, nor in most cases easy to 

 obtain ; and the means of displaying this operation 

 with tolerable certainty may perhaps be interesting 

 to some of the readers of Science-Gossip. 



