654 



MYRIAPODA. 



Fig. 318. 



elapsed before the body of the young my- 

 riapod was so far liberated as to remain only 

 partially enclosed between the two halves of the 

 shell, as represented in Jig. 318, being still at- 

 tached to its interior by a pe- 

 dicle or funis (Jig. 319, d). 

 So remarkable is its condi- 

 tion at this period that it 

 strongly resembles the ex- 

 pansion of the germ in the 

 seed of a plant, rather than 

 the evolution of a living 

 animal. The embryo is per- 

 fectly motionless, and the 

 bursting of its shell appears 

 to be effected not by any 

 direct effort of its own, since, up to this time, it 

 has acquired only the form and external sem- 

 blance of a living animal ; but, by the force of ex- 

 pansion of the growing body, the development 

 of which being greatest along the dorsal or larger 

 curvature, exerts, in consequence, a greater de- 

 gree of force against the middle of the dorsal 

 than the corresponding part of the ventral sur- 

 face ; the head and tail of the embryo acting 

 as a fulcrum against the ventral surface only at 

 the ends of the shell, and thus bending it into 

 the kidney-shaped form it assumes, while the 

 dorsal surface of the embryo is gradually 

 pressed through the opening. From the com- 

 parative rapidity of its enlargement imme- 

 diately after the shell is fissured, Mr. Newport 

 observes, that it seems as if the stimulus given 

 to it by exposure to a new medium, atmo- 

 spheric air, were the great means of exciting its 

 evolution. 



The embryo is now formed of eight distinct 

 segments (Jig- 319), including the head, the 

 ninth or anal segment being still indistinct. 

 The head is more 

 denned in its out- 

 line, and firmer in 

 texture than other 

 parts of the body, 

 and is inflected 

 against the under 

 surface of the pro- 

 thorax (2) or second 

 egment, from which 

 it is divided on the 

 upper part by a deep 

 transverse line : at 

 its sides it exhibits 

 a faint trace of the future antennse. The four tho- 

 racic segments also exhibit on their ventral sur- 

 face little nipple-shaped extensions, three of 

 which on each side are the rudiments of future 

 le<*s. When viewed from above, the body of the 

 embryo appears compressed and wedge-shaped, 

 its greatest diameter being in the second and 

 third segments, while each succeeding segment 

 is more and more contracted. Mr. Newport 

 was unable at this period to detect any separate 

 internal organs, the whole embryo being still a 

 congeries of vesicles or cells, in the midst of 

 which there seemed to be some faint traces of 

 the commencement of an alimentary canal. 



" At the end of the first day," continues Mr. 

 Newport," I carefully removed the embryo and 

 shell into diluted spirits of wine, and, on ex- 



Fig. 319. 



amination beneath the microscope, found the 

 body of the embryo covered with an exceed- 

 ingly delicate cuticle, through which the cells 

 it was formed of were distinctly visible. It 

 was also completely enclosed in a smooth and 

 perfectly transparent membrane (Jig. 319, c), 

 which seemed to contain a clear fluid, inter- 

 posed between it and the embryo. This mem- 

 brane I regard as the analogue of the amnion, 

 the vitelline or investing membrane of the 

 embryo in the higher animals, and identical 

 with the membrana vitelli, before described, as 

 the proper membrane of the yelk in the egg of 

 Julus. It is a shut sac that completely invests 

 the embryo, except at its funnel-shaped ter- 

 mination at the extremity of the body (fig. 319, 

 d), where it is constricted, and together with 

 another membrane (e), which in the unburst 

 egg is external to this and lines the interior of 

 the shell, assists to form the cord or proper 

 funis (d). 



"The funis enters the body of the embryo at 

 the posterior part of the dorsal surface of the 

 future penultimate segment, where the mucro 

 or spine exists in the adult animal, and not at 

 the dorsal surface of the thoracic region, as seen 

 by Rathke in the Crustacea. The proper anal 

 or terminal segment is, as yet, but im- 

 perfectly developed. In the funis (d), I 

 also observed some exceedingly delicate struc- 

 tures that exhibited all the appearance of 

 vessels. They seemed to enter the body by 

 two sets, that were spread over and entirely 

 lost in the membrane (e). Whether these were 

 indeed vessels, or merely folds of the mem- 

 brane I am not certain. The membrane (e} in 

 which they appeared adheres closely to the 

 shell and retains the embryo in connection 

 with it by means of the funis. In the unburst 

 egg, this is also a shut sac like the amnion, 

 and forms the membrana externa or chorion (?), 

 the second or outer investing membrane of the 

 ovum lining the interior of the shell. 



" The detection of these two investing mem- 

 branes of the embryo in Myriapoda may,' 1 says 

 Mr. Newport," be regarded with some interest 

 in reference to the analogies which they bear 

 to similar structures in Vertebrata, since they 

 shew the persistence of one universal law in the 

 mode of development of the germ." 



On the third day the embryo had consider- 

 ably increased in size, but was still perfectly 

 motionless and attached to the shell by the 

 funis. This attachment continues for many 

 days, during which the embryo remains par- 

 tially protected by the two halves of the shell. 

 When examined at this period in the recent 

 state, all the parts of its body are still indis- 

 tinct, but in specimens that have been for some 

 time in 'spirits of wine, the divisions between 

 the segments are well marked. The rudiments 

 of the legs are more developed, but those of 

 the second and third segments less than the 

 fifth, so that not only at the bursting of its 

 shell, as noticed by Savi, but also for several 

 days afterwards the embryo is competely apo- 

 dal, the future limbs existing only in a rudi- 

 mentary state. Posteriorly to the fifth segment, 

 the body is more soft and delicate, and the seg- 

 ments less clearly defined. This results from 



