THE HEAD PROBLEM 



6ii 



of this assumption has been persistently challenged, and experience has 

 demonstrated that it should not be pressed too far. In a very general 

 way, however, the law has proved helpful, especially in relation to the 

 head problem. 



In 1878 Francis Balfour made the important discovery that in elasmo- 

 branch embryos the coelom extends continuously throughout head and 

 trunk. He thus demonstrated that in this fundamental respect head 

 and trunk are similar to a degree not previously realized. Balfour also 

 stressed the fact that, by the formation of gill-pouches, the cephalic 

 coelom is divided into a series of mesodermal "cavities," which Balfour 



CEREBELLUM 



HEAD CAVITIES 

 Y 



MOUTH 

 HYPOPHYSIS 

 OPTIC RECESS 



^THYROID 



Fig. 505. — A parasagittal section of a squalus embryo showing the three "head 

 cavities" from which the six eye muscles develop. The somatic motor nerve relations 

 are also shown. (After Neumayer modified.) 



erroneously compared with trunk somites. The falsity of this comparison 

 was subsequently recognized when it was demonstrated that the trunk 

 somites develop from the epimere, while the mesoderm of the visceral 

 arches comes from the hypomere. 



In 1 881, Balfour's pupil, Marshall, emphasized the distinction between 

 the segmentation of the somites and that of the mesoderm of the visceral 

 arches, and asserted the independence of the two. Only segmentation 

 of the dorsal somites of head and trunk are comparable. The mesodermal 

 cavities within the visceral arches are peculiar to the head. This contrast 

 between "mesomerism" and "branchiomerism" has been recognized by 

 later morphologists. Whether or not these two kinds of segmentation 

 originally corresponded has remained a controverted issue to this day. 

 Marshall also showed that the first or premandibular "cavity" gives 

 rise to the four eye muscles innervated by the oculomotor nerve, while 

 the external rectus muscle comes from the third somite. 



