EMBRYOLOGY OF LIMULUS POLYPHEMUS. 215 



At this stage the periphery of the egg (Fig. 76) shows a separation of the external 

 layer or blastodermic cells (e) into protoderm or amnion (p) and chorion {ch). 



" So far as I am aware (writes Dr. Packard) Apus is the only other Crustacean in 

 which a ' protoderm ' like that of Limulus is formed. It is not comparable with the 

 nauplian or larval skin, as it is composed of distinct cells and is entirely circular, with no 

 lateral diverticula surrounding the appendages." 



Fritz Miiller has pointed out 1 that in some Isopoda (as for instance in Philoscia) the 

 larval skin is not only without any folds or sac-like diverticula, but is closely applied to 

 the egg-membrane. 



Certainly when the embryo Limulus first appears its position is the same as in Asettus, 

 Ligia, Philoscia, and other Isopods, i. e. with its ventral surface convex 2 (see Woodcut, 

 fig. 77 and PL XXXIII, figs. 1 and 2). 



The egg at this stage has two membranous envelopes, a dense inelastic 'chorion ' and 

 an inner elastic 'protoderm.' 3 This chorion remains entire so long as development is 

 arrested or is sluggish, but as soon as the embryo increases in size, the tough inelastic 

 chorion splits asunder, and the inner elastic protoderm enlarges, becomes dense, and 

 vicariously fulfils the duties of the former. 



Dr. Packard thinks that no particular taxonomic importance can be attached to the 

 presence of this inner membrane, but that it is simply a biological matter having reference 

 to the circumstances under which the eggs are deposited. Pie reminds us that the eggs 

 of Limulus are laid loose in the sand and remain there, or are washed about by the waves 

 during a month or six weeks, and possibly over a whole season. 



In the case of Apus (whose ova are similarly provided with a chorion and protodermal 

 envelope) the female lets her eggs fall at random to the bottom of the water, where they 

 remain until hatched by the sun's warmth — a process which occupies two or three weeks. 

 The chorion splits as in Limulus, and during the subsequent changes of the embryo the 

 semi-transparent inner egg-membrane (' protoderm ') takes its place. 



Whilst fully admitting the undoubted utility and fitness of this duplex envelope 

 which, like the ephippial capsules of Daphnia, may have to undergo the severest trials in 

 protecting the fertilised ovum within, often through protracted periods of delayed 

 exclusion ; it nevertheless seems only philosophical to homologise if possible these 

 embryonal investments of the young of the several orders of Crustacea. For although so 

 ephemeral in their nature and varying in character according to the conditions under 

 which the young are placed 4 they help us, as Claus, Darwin, Fritz Miiller, and other 



1 ' Facts and Arguments for Darwin,' translated from the German by W. S. Dallas, F.L.S., London, 

 1869, 8vo, p. 71. 



2 H. Woodward, article "Crustacea," p. 649, vol. vi, 'Encyclopaedia Britannica,' 9th edition, 4to, 



1877. 



3 Dohrn calls the inner membrane in the egg of Limulus the " chorion," and the outer the "exochorion," 

 but Packard's term protoderm appears preferable. 



* Many of the Amphipoda and Isopoda display the most assiduous care for their young, and, like 



