XOTES. 175 



metry, ;ind the recent researches of E. B. Wilsox and Lwoff lead 

 to the concUision that the so-called meso/'/astic pok-cc/ls, which 

 were described by Hatschek, have no real independent existence. 



3. (p. 123.) Whether the dorsal and ventral fin-spaces are 

 actually derived from the original mvoceel, as described hv Hat- 

 schek, or do not rather arise by a splitting of an originallv solitl 

 thickening of the gelatinous connectixe tissue which surrounds 

 them, must remain doubtful. The cavity of the metapleural folds 

 certainly arises as a sthlzoiai, i.e. by a hollowing out of a solid 

 thickening. Even in case the fin-spaces also arise as schizoctels, 

 Hatschek's interpretation of their morphological significance might 

 still hold good. 



4. (p. 123.) A transitory pouch-like diverticulum of the myo- 

 ccel has been observed in connexion with the formation of the 

 sclerotome in the Selachian embryo by Rai;l and H. E. Ziegler. 



5. (p. 129.) Since the work of B.-vlfour on the development 

 of Elasmobranch fishes (Selachians), it has been known that the 

 paired prajmandibular head-caxities communicate with one another 

 across the median line in the embrx'o. The important results 

 obtained by the researches of Kupffer (Petromyzon, .Acipenser), 

 Kastschkxko (Selachian), and Julia Pi-att (Selachian), not only 

 established the fact that the prKmandibular cavities arose essen- 

 tially as anterior archenteric pouches (cf. Fig. 72), but also that 

 the median cavity which eflected their communication across the 

 middle line, from side to side, arose by constriction from the front 

 end of the archenteron (using the latter term with some latitude), 

 and that, therefore, the tiuioii of ill c rigiii aiui lift p)-u)>iaiuiibiilar 

 cai'itici ill iiic cinhrxo of tlw crania to Vortol'ratos is friiiiarx, and 

 not sOiOiiiiarv. as icas prorioi/slv sn/^posoJ. 



For an excellent historical and critical summary of our knowl- 

 edge of the origin of the head-cavities in the craniate ^'ertebrates, 

 the reader may consult Fkhriep. (See bibliography.) 



6. (p. 130.) The ciliation of the ectoderm in the larva of 

 Amphioxiis continuing, as it does, long after the muscles have been 

 fully dift'erentiated, and when the cilia are therefore no longer 

 required for purposes of locomotion, should be especially noted as 

 evidence of a verv archaic organisation. 



^\'e shall find in the last chapter that the possession of a ciliated 



