540 BASIC FEATURES OF VERTEBRATE MORPHOGENESIS 



primitive segmental structure of the vertebrate brain. It is to be observed that 

 the more conservative figure 253, taken from Goodrich, does not emphasize 

 neuromeres, for, as observed by Kingsbury ('26, p. 85), the evidence is over- 

 whelmingly against such an interpretation. The association of the cranial nerves 

 with the gill (branchial) region and the head somites, shown in figure 253, 

 will be discussed further in Chapter 19. 



A third factor which awakened curiosity, concerning the segmental theory 

 of head development, is branchiomerism. The latter term is applied to the 

 development of a series of homologous structures, segmentally arranged, in 

 the branchial region; these structures are the visceral arches and branchial 

 pouches referred to above. As mentioned there, the branchial pouches or out- 

 pocketings of the entoderm interrupt a non-segmented mass of lateral plate 

 (hypomeric) mesoderm, and this mesoderm secondarily becomes segmented 

 and located within the visceral arches. These arches when formed, other than 

 possibly the mandibular and the hyoid arches (fig. 253), do not correspond 

 with the dorsal somitic series. Consequently, "branchiomerism does not, there- 

 fore, coincide with somitic metamerism." (See Kingsbury, '26, p. 106.) 



Undoubtedly, much so-called "evidence" has been accumulated to support 

 a theory of head segmentation. A considerable portion of this evidence ap- 

 parently is concerned more with segmentation as an end in itself than with a 

 frank appraisal of actual developmental conditions present in the head (Kings- 

 bury and Adelmann, '24 and Kingsbury, '26). However, the evidence which 

 does resist critical scrutiny is the presence of the head somites which includes 

 the pre -otic somites and the first three or four post-otic somites. While the 

 pre-otic somites are somewhat blurred and slurred over in their development 

 in many higher vertebrates, the fact of their presence in elasmobranch fishes 

 is indisputable and consistent with a conception of primitive head segmentation. 



Furthermore, aside from a possible relationship with head-segmentation 

 phenomena, the appearance of the pre-otic and post-otic head somites coin- 

 cides with basic developmental tendencies. As observed above, for example, 

 there is a tendency for nature to use generalized developmental procedures in 

 the early development of large groups of animals (see von Baer's laws, p. 522, 

 and also discussion relative to Haeckel's biogenetic law in Chap. 7). Nature, 

 in other words, is utilitarian, and one can be quite certain that if general 

 developmental procedures are used, they will prove most efficient when all 

 factors are considered. At the same time, while generalized procedures may 

 be used, nature does not hesitate to mar or elide parts of procedures when 

 needed to serve a particular end. The obliteration of developmental steps 

 during development is shown in the early development of the mesoderm in 

 the vertebrate group compared to that which occurs in Amphioxus. In the 

 vertebrate embryo, as observed previously, the hypomeric mesoderm is un- 

 segmented except in a secondary way and in a restricted area as occurs in 

 branchiomerism. However, in Amphioxus, early segmentation of the meso- 



