No. 2.] THE EMBRYOLOGY OF LIMULUS. 233 



confirmed the early observation of Weismann ('63) that the 

 Hexapod antennae are postoral, and have shown that they are 

 innervated from the second or deutocerebral neuromere. 

 Between this and the mandibular ganglion is the third com- 

 ponent of the brain, the tritocerebrum, and in a few forms this 

 has been shown to have a small embryonic appendage which 

 apparently becomes obsolete in the later stages. This has 

 been observed by Tichomiroff (teste Cholodkowsky i) in the 

 silk-worm ; by Carriere ('9o) in the bee Chalicoderma ^ while in 

 a note Dr. Wheeler sends me an account and a drawing of the 

 embryo of the Collembolan, AmuHda maritima, in which the 

 appendage between the antenna and the mandible, the trito- 

 cerebral appendage, is well marked. Apparently Wheeler had 

 seen traces of the same in Doryphora ('89, p. 337 Fig. 44). 

 Regarding these neuromeres and appendages the conclusion is 

 inevitable that they belong to the primitively postoral series. 

 They arise in the same line and agree in all respects with those 

 further back. The only other supposition would be that they 

 are preoral and temporarily wander backwards to be immedi- 

 ately returned to their proper position, a supposition of very 

 doubtful value. In the light of these observations we must 

 regard the Hexapod head as composed of at least six elements, 

 the procephalic lobes and five postoral somites, each of the 

 latter having appendages. 



In the Arachnida our evidence is much less abundant and 

 much less detailed. Schimkewitsch ('89) describes in the spiders 

 ocular and rostral ganglia (PI. XXI, Fig. 3) in advance of the 

 ganglion of the cheliceras, while in the schematic Fig. 5 of his 

 PI. XXIII he represents the cerebral ganglion (in front of the 

 rostral ganglion) as three-lobed. Patten ('90) describes the 

 brain of the scorpion as composed of three pairs of ganglia,^ 

 while Jawonowsky ('92) figures four postoral somites in front of 

 the cheliceral somite, the posterior of which bears a pair of 



1 In his preliminary paper Tichomiroff merely says " Es existirt bei dem Seide- 

 wurm eine echte untere Lippe . . . und die als der allbekannten Oberlippe der 

 Insecten homolog betrachtet werden darf." His later paper is unfortunately 

 buried in an outlandish tongue. 

 . 2 Carriere recognizes y^wr cerebral somites and finds a preantennal appendage. 



3 See also Metschnikoff ('70), PI. XV, P^ig. 14. 



