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pleure just below the 8th—12th post-otic somites!). It is seen by 
--comparison that the pronephros in Petromyzon lies as a whole anterior 
to the pronephros in Squalus. The cause, or causes, therefore, which 
have produced segmental changes in these two forms have not acted 
in the same direction upon the pronephros and the ventral longitudinal 
muscle of the branchial region. As to its pronephros Petromyzon 
seems to occupy an intermediate position between Amphioxus and 
Squalus. The homologues of the pronephric tubules in Amphioxus 
(Bovert, P. Mayer, RÜCKERT) exist only in the branchial region. In 
Petromyzon they are limited to four segments lying immediately behind 
the branchial basket in early stages of development. In Squalus the 
branchial basket and the pronephros are still further reduced and the 
latter more posteriorly situated. Thus as the result of the division 
of labor, the respiratory function become more concentrated anteriorly 
and the excretory function in the opposite direction. Reduction of 
the organs of these two functions therefore takes place in opposite 
directions. 
On the other hand, the differences between Petromyzon and Squalus 
as regards the parietal muscular system would indicate that concom- 
itantly with the reduction in the branchial basket there has "been an 
extension of this muscular system. van WrisHE (’89, p. 470) writes: 
“Beim Amphioxus wachsen die dariiber befindlichen Myotome mit 
ihrem ganzen ventralen Ende in die Falte hinein, welche später großen- 
teils die Peribranchialhöhle lateral begrenzt. Bei den Cranioten da- 
gegen senden die Myotome, wie hauptsächlich von Donrn beschrieben, 
nur Knospen in die Gliedmaßenfalte und kommen dann zwischen Cölonı 
und Haut zusammen. Die Cranioten haben sich also von 
den Acranien getrennt, ehe die parietale Musculatur 
den Körper ventral umwachsen hatte.” If I understand 
vAN WIJHE correctly, he would compare the ventral growth of the 
myotomes of Amphioxus into the fold which confines laterally the 
peribranchial space with the growth of the myotomes of Craniota into 
the ectodermal fold which forms the Anlage of the paired appendages 
— and with this growth alone. While I believe that van WiJHE is 
right in holding that in the growth of myotomic buds peripherad into 
the paired appendages and in the growth of such mediad and ventrad 
we have to do with ontogenetic processes of different morphological 
1) van WisHe (89) and Rast (’96) agree that the first of the pro- 
nephric tubules is differentiated in Pristiurus below the 8th post-otie 
somite, 
30 * 
