PLATE 9: EXPLANATION OF FIGURES CcONTINUED) 



76 A diagram showing the greatly modified metameric relations of the eye 

 muscle nerves, based upon the conditions found in Squalus embryos. The dia- 

 gram is a composite of different stages of development. The relations of the 

 oculomotorius nerve to the ramus ophthalmicus profundus trigemini are those 

 which appear in a 12 mm. embryo, while those of the trochlearis to the ramus 

 superficialis are such as appear in a 25 mm. embryo. The ramus recurrens of 

 the abducens appears in a 19 mm. embryo. The neuromeres and somites are 

 as they appear in much earlier stages of development. The diagram is not in- 

 tended to show any hypothetical stage in the phylogeny of vertebrates, but 

 merely to present gra])hically the highly modified metameric relations of the eye 

 muscle nerves to the primary neuromeres and the somites of VanWijhe. 



77 A diagram of the left lateral aspect of the head of a Petromyzon embryo 

 of 50 mm., showing the relations of the post-otic myotomes and their associated 

 ventral nerves. The innervation of the first three post-otic myotomes (4, 5, 6) 

 by the ventral nerves of the ventral nerves of the fourth and fifth post-otic 

 myotomes (7, 8) is especially to be noted, as is, also, the splitting of the first 

 post-otic myotome (4) along the dorso-lateral line into a dorsal and ventral 

 division. The homology of the first post-otic somite of Petromyzon with the 

 fourth somite of Squalus is based on the evidence given by Koltzoff ('02). 



78 A diagram based upon a reconstruction of a 3.5 mm. embryo of Petro- 

 myzon by KoltzofT ('02) showing the splitting of the anterior post-otic myotomes 

 into median and lateral divisions. In order to explain the present relations of 

 the eye-muscle nerves (fig. 76) to the pre-otic myotomes in Squalus, a similar 

 subdivision of the myotomes of Van Wij he's first, second and third somites may 

 be assumed. 



79 A diagram based upon a cross-section of a Petromyzon embryo of 3.5 mm., 

 showing the three divisions (my. 4 m., my. 4 d. 1., my. 4v. 1.) of the first post-otic 

 myotome, and their relations to the otic capsule. The hypothetical primitive 

 connection of a somatic motor nerve with the median (transient) division of the 

 myotome is shown. The two divisions of the myotome lateral to the otic cap- 

 sule are permanent and become innervated by the ventral nerves of the fourth 

 and fifth post-otic myotomes. By a similar substitution, it may be inferred, 

 the rectus posterior muscle — a pre-otic muscle — has become innervated by the 

 abducens — a post-otic nerve. 



80 A diagram based upon a frontal section in the occipital region of a Petro- 

 myzon embryo of 4 mm., showing the relations of the anterior most post-otic 

 ventral nerves to the myotomes. The division of these myotomes into a median 

 portion and a portion lateral to the otic capsule is shown. 



81 A diagram of the left lateral aspect of a Squalus embryo, showing the 

 hypothetical primitive relations of the eye muscles to the lateral trunk muscu- 

 lature. Those myotomic divisions which are not functional in the adult Squalus 

 are indicated by broken lines. Only those pre-otic myotomes which have ac- 

 quired connection with the bulbus oculi and have become functional as eye mus- 

 cles have persisted. The homologues of myotomes 4, 5 and 6, however, are 

 functional in Amphioxus and Petromyzonts. 



82 A diagram intended to show the conditions under which the dorsal chiasma 

 of the trochlearis has been acquired. The left side of the diagram shows an 

 hypothetical stage when the myotome of VanWijhe's second somite was split 

 into three moieties. The right side of the diagram corresponds in all essential 

 respects with the conditions found in a 25 mm. Squalus embryo. For an inter- 

 pretation of the figure, see p. 114 of the text. 



186 



