160 A. S. PACKARD, JR., ON THE 



embryonic phenomena (t. e., the formation of the blastoderm) of Asellus as well as Crangou, 

 are entirely like those which have been observed in the Aphides by Metznikow, in the 

 Arachnids by Claparede, in the fresh water Gammari and in a great number of the Arthro- 

 pods," and we may now add in Limulus. 



The protoderm (" amnion," "parietal membrane" of Brandt) as Bessels aptly terms it, 

 is not moulted until after the primitive disk and the rudiments of the six pairs of cephalic 

 limbs appear. In about a week after the protoderm appears, the primitive disk (fig. 10, ^m,) 

 is formed, as easily seen, by a local multiplication of the blastoderm cells on one side of 

 the egg. (Fig. 9 represents a portion of the primitive disk consisting of protodermic 

 cells resting on the yolk.) The protoderm or peripheral layer of these blastoderm cells 

 (which form a thin bed enveloping the yolk, as I could easily trace the thin layer around 

 the whole circumference of the egg ^) begins to peel off at the depression or re-entering 

 angle between the end of the abdomen and the yolk mass (figs. 12 a, 12 b, a). It grad- 

 ually loosens as the rudimentary limbs become longer, and as they increase in length the 

 protoderm peels off still more, leaving a clear space between it and the embryo, as seen in 



fig- 13. 



The protoderm, then, in Limulus, though not moulted until after the formation of the 

 primitive disk (the equivalent of the primitive band in Crustacea and insects) is a simple 

 sac, and exactly homologous with the protoderm of insects. Though I could detect no 

 nuclei in the protodermic cells, it will be remembered that they were easily seen in the 

 primitive blastodermic cells (fig. 6, 6 a). This protoderm is evidently homologous Avith 

 the protoderm of A2nis longicaudatus Leconte, observed in alcoholic specimens. In this 

 genus the chorion is thick, structureless, wdth irregular laminae as in Limulus. The 

 protoderm in eggs in which the germ has not yet appeared, though the yolk has some- 

 what contracted, presents exactly the appearance in outline as in Limulus (fig. 9, p), the 

 cells being in profile spherical and projecting out in the same manner. In a vertical view 

 they are rounded, more or less angular when crowded ; in some cases with a nucleus, and 

 in others not, so that the disappearance of tlie nucleus in the protodermic cells of Limu- 

 lus is not apparently of great importance. 



The protoderm of more advanced eggs just before the chorion splits, as seen in fig. 11 

 (11 a, profile view), becomes tough and dense. On a vertical view it is seen to be composed 

 of irregular liexngonal cells, which are sometimes five-sided, and rarely four-sided ; 

 scarcely any two cells being alike (fig. 11). The walls of the cells are double, and are 

 either strongly waved, or each side (sometimes but two or three) has from three to five 

 long, slender projections, with the ends sometimes knobbed, directed inwards. The cells 

 are usually packed closely together, but are occasionally separated by quite a wide inter- 

 space. 



Having shown Ihe homology of this protoderm in Limulus with that of the insects, let us 

 examine its relation with the blastodermic skin moulted in other Crustacea. And here I 

 may remark that the admira])le and exhaustive papers on the embryology of the Crustacea 



1 E. Van Beneden shows in the eggs of Aselhis that the dorsal side of the egg. But this opinion should be aban- 



blastodermio cells entirely surround the egg. The dorsal doned. In all, the blastoderm extends over the wliole sur- 



cells by their flatness and extreme delicacy are easily over- face of the egg before the appearance of the rudiments of 



looked; and from this reason it was thought for a long time the appendages. Kecherches sur I'Embryogenie des Crus- 



thatthe Arthropods in general, and especially the Crustacea, taces. Bulletins de I'Acad. Roy. Belgique, xxviii, 18C9. 



were deprived of a blastoderm on what is destined to be the 



