194 BULLETIN : MUSEUM OF COMPAKATIYE ZOOLOGY. 



The contention that the constrictions between van Wijhe's somites are 

 incomplete does not appear to me to militate greatly against the view 

 that they have morphological value, inasmuch as their permanency has 

 been repeatedly attested (van Wijlie, Hoffmann, Xeal, and SewertzofF). 

 Xor does Eabl apparently consider this argument as of great weight, 

 since he regards van Wijhe's 5th (1st post-otic) somite — though the 

 constrictions which are found in front and behind are incomplete — as 

 a true somite. The reduction in the myotomic portion of the dorsal 

 mesoderm accounts in great part for the incompleteness of the constric- 

 tions. I believe that one who follows the development of the pre-otic 

 and sub-otic mesoderm in Ammocoetes, and observes the ontogenetic 

 dissolution of the compact dorsal mesoderm into loose mesenchyma, 

 which follows the great enlargement of the nerve ganglia and of the 

 otic capsule, is in a position to understand the reduction of the dorsal 

 mesoderm in this region in Yertebrates higher in the phylogenetic scale 

 than Ammocoetes.^ 



wick ('92), that this is not true of the mesoderm segments discovered by Dohrn 

 ('90, '90^) in that form. Dolirn apparently did not endeavor to ascertain whether 

 they were symmetrical or not. I am unable to determine, even in carefully made 

 reconstructions of well oriented frontal sections of embryos at the same stage of 

 development as that described and figured by Dohrn ('90^), whether or not there 

 is a correspondence of the mesodermal segments on the two sides of the head an- 

 terior to the one which, in my opinion, corresponds with the loth segment of Dohrn. 

 While my own negative conclusions cannot be regarded as in any sense disproof of 

 the segmental value of Dohrn's somites, it is my opinion that the evidence of their 

 variability shown by the conflicting results of Killian ('91) tends to throw consider- 

 able doubt upon it. Since Killian ('91, p. 103) finds that of the anterior of these 

 segments one is to be regarded as the sclerotome portion of a somite, while others 

 are simply vesicular enlargements of the mesoderm of the mandibular arch, it is 

 to be inferred that Dohrn subjected the head somites of Torpedo to little critical 

 examination. To regard as evidence of somites all vacuolar spaces in the dorsal 

 (and lateral !) mesoderm which appear between the sornatopleure and splanchno- 

 pleure at the time these layers separate, seems to be too uncritical. Similar phe- 

 nomena appear in the mesoderm of Squalus in those early stages of development, 

 when the ccelom is in the process of formation, viz. in stages when the neural plate 

 is widely expanded and the embryo possesses 4 or 5 somites. Recent studies by 

 SewertzofI ('98^ render still more doubtful the results of Dohrn and Killian. 



1 A mechanical explanation of the constrictions between the head somites of 

 van Wijhe, such as that offered, but without evidence, by Kastschenko ('88), 

 seems hardly worthy of consideration. That such constrictions as those, for ex- 

 ample, between somites 3 and 4, and 4 and 5, cannot result from the so called 

 mechanical influence of visceral clefts, follows from the evidence already stated by 

 Hoff"mann ('94 and '96) that in Squalus, the constrictions lie dorsal to the visceral 

 arches. I cannot, however, agree with Hoffmann that we may conclude from this 

 evidence that the visceral arches are intersomitic in position, as are the ribs in the 



