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



The freshly hatched young of Artemia, and probably all the Branchiopods, is at first sur- 

 rounded by what Joly describes as a "membrane mince transparente." This seems homolo- 

 gous with the amnion of Isosoma, which I have noticed in the Memoirs of the Peabody 

 Academy of Science, 1870. That was structureless, and it should be determined how far 

 these amnion cells are absorbed in certain arthropods, as they seem to be in Crustacea gen- 

 erally, so far as our present knowledge extends. 



Formation of the embryo. Between one and two weeks after the eggs are laid the 

 primitive disk appears, bearing six pairs of appendages. Supposing that the germ ap- 

 peared first as a simple, pale spot, and that the appendages appeared successively in pairs 

 as in Astacus Jluviatilis, Palcemon adspersus, and EripJiia spinifrons (observed by 

 Rathke), I watched eagerly for two seasons, and examined at least over a thousand eggs, 

 endeavoring to detect a primitive disk, without traces of appendages ; but, probably owing 

 to the opacity of the egg, with no success ; though a few eggs the first season presented 

 certain appearances which led me to suppose them to be the germ, some with one, others with 

 three pairs of limbs. (See p. 159, note.^) The earliest appearance of the primitive band, or 

 disk, is indicated by figs. 10 and 12. At this period the protozonites are faintly indicated, 

 each bearing a pair of appendages forming flattened bands, which appear as if pressed down 

 into the yolk, while the abdomen is indicated by a thickening of the posterior third of the 

 oval disk, being paler than the rest of the body. The median line of the body is also seen 

 at the same time. The germ is regularly oval, one-half as wide as the egg, and about two- 

 thirds as long. Around the edge is a pale areola, destined to be the lower edge of the 

 carapace ; it is most distinct along the middle, merging imperceptibly into the anterior and 

 posterior end of the disk. It is a thin ridge, due to a local heaping up of cells. The six 

 pairs of appendages, at first forming simple bands, gradually increase in size towards the 

 last pair. The first pair are much smaller than the other five and are situated considerably 

 behind the mouth, which is very distinct. The abdomen is separated by a slight inter- 

 space from the preceding protozonite, while the areola merges imperceptibly into the abdo- 

 men. There are no traces of abdominal appendages. 



At this time the disk is very flat, scarcely rising above the surface, and in profile is indi- 

 cated by fig. 10. It is evidently due to the multiplication and heaping up on one side of 

 the egg of the blastodermic cells, the layer forming the disk thinning out and surrounding 

 the rest of the egg, as I could easily follow it around the entire circumference of the egg. 

 Fig. 9 represents a portion of the disk, showing the layers of blastodermic cells (the outer 

 layer of which is destined to split off and form the protoderm) resting on the yolk. The 

 primitive disk in Limulus arises just as in Crustacea generally, as I have seen it in Lupa, 

 Hippa, Idotea, and as described by other observers in other Crustacea ; the great apparent 

 differences in Limulus being probably at least due to the small size of the yolk granules, 

 and consequent opacity of the egg, and to the small size and large number of the blasto- 

 dermic cells, a circumstance of minor importance. Indeed the general features of the 

 formation of the primitive band, of the protozonites, and the appendages of Limulus, ac- 

 cord not only with those of other Crustacea, but also strikingly with the Arachnida, and 

 in only a less degree with the Insecta. 



When the embryo is three days older (fig. 12 upper and lower figures from the same 

 egg, 12 a, 12 h,) it is seen more distinctly, the disk becoming slightly thicker, especially 

 in the abdominal portion. The blastoderm is thickest over the end of the abdomen. 



