No. I.] THE EMBRYOLOGY OF LIMULUS. 47 



Blastoderm Cuticle. 



With the formation of the blastoderm, the blastodermic cuticle 

 is first laid down. Its history need not be given here, as I have 

 already ('85, p. 524) detailed it, and have suggested for it and sim- 

 ilar envelopes, Claparede's term "deutovum." The occurrence 

 of these Blastodermhantc7i is frequent in the Arthropod phylum. 

 In Limulus the envelope persists as a protective structure until 

 a late stage in development, but it is omitted from my figures. 



Early External Development. 



Stage C. — At from six to eight days after impregnation a 

 lighter patch is visible on one side of the &g^. Its outline is 

 not distinct, but in general it may be said to be circular. The 

 change which this undergoes in two or three days (eight to 

 eleven days from impregnation) is slight ; at the latter date a 

 pit is apparently seen in the centre of the white spot (Fig. 21). 



For this lighter patch I have taken the same name which 

 was given by Claparede ('62) to a similar structure in the develop- 

 ing Arachnid ^g^} The spot is the first appearance of what is 

 to form the primitive streak. At first this spot is circular, 

 but it soon becomes elongate. The next day a second cloud 

 appears immediately adjoining the first and connected with it 

 (Fig. 22). I am not positive in my identification, but believe 

 that the primitive cumulus marks the anterior end of the 

 embryo. At first the posterior, or secondary, cloud is smaller 

 than the primitive cumulus, but it rapidly increases in size, 

 while its outlines become more indistinct than shown in Fig. 23. 

 At the same time the primitive streak extends backward from 

 the spot mentioned above, into the posterior cloud ; the anterior 

 spot remaining the widest of the whole. For reasons which 

 will appear farther on I regard the widest end of the primitive 

 streak as marking the position of the future mouth. The 

 posterior cloud continues to grow until the result is as shown 

 in Fig. 24. 



Next there appears a transverse line behind the primitive 

 cumulus, cutting the embryo into an anterior, or cephalic 



^ I retain this term, ' primitive cumulus,' notwithstanding Kishinouye ('90) has 

 shown that it is possible that Claparede has mistaken the order of appearance of 

 his " cumulus primitif " and the ' calotte.' 



