308 H. V. NEAL 



Fijis. 14 to 17 represent a sei'ies of Chordate embryos showing the increasing 

 conspicuousness of the 'neuromeres' and the associated reduction of somites in 

 the head region. The evidence favors the conclusion that neuromeres are a 

 coenogenetic acquisition of the Chordates. The mesodermic segmentation, 

 however, bears the earmarks of an ancestral segmentation which degenerates 

 during the phylogenesis of the Chordata. 



Fig. 14 A larval Amphioxus after Hatschek ('81), showing the unbroken 

 series of mesodermic cavities, from each of which is differentiated a permanent 

 myotome. 'Neuromeres' are wanting. If 'neuromeres' were an ancestral char- 

 acteristic like the mesomeres, how is the absence of the former and the presence 

 of the latter to be explained? None of those who have assumed the metameric 

 importance of the neuromeres have ever given an answer to this important 

 question. 



Fig. 15 A diagram of larval Petromyzon, showing the continuous series of 

 myotomic divisions, each of which, as in Amphioxus, forms a permanent myo- 

 tome and persists in the adult (Koltzoff, '02). In the diagram the somites are 

 represented as they appear in an eight-day (Naples) embryo, while the neuro- 

 meres appear (greatly exaggerated in the diagram) as in a twelve-day embryo. 

 In later stages neuromere III secondarily subdivides into two segments — 'rhom- 

 bomeres' 1 and 2 auctorum. The reason why the pririiary segment and not its 

 secondary subdivisions is considered to be a true neuromere is given in the 

 context. 



Fig. 16 A parasagittal section of a 4-mm. Squalus embryo, showing the 

 seven anterior primary brain segments (considered by the writer as the true 

 Vertebrate 'neuromeres') and the series of mesodermic somites — the exact homo- 

 logues of the series shown in Amphioxus and Petromj^zon. As in the latter ani- 

 mal, neuromere III subdivides secondarily to form the 'rhombomeres' 1 and 2 

 (see figs. 1 and 2). From all of the somites, except somites 4 and 5, myotomes are 

 difTerentiated. The myotome of somite 6 is transient, however, and that of 

 somite 7 forms the first permanent postotic myotome. The myotomes of somites 

 1 to 3 form the ej-e-muscles (Neal, '18). Yet, while the degeneration of cephalic 

 myotomes has thus liegun in Elasmobranchs, the 'neuromeres' are much more 

 conspicuous than in Cyclostomes. 



Fig. 17 A parasagittal section of a chick embryo of 13 or 14 somites, showing 

 the conspicuous neuromeres (I to VII), homologous with those of Petromyzbn 

 and Squalus, and the disappearance of true somites anterior to the seventh 

 {myP). A noteworthy fact is the later secondary sul)division of 'neuromere' III 

 into two segments, the exact homologues of 'rhomJiomeres' 1 and 2 of Petro- 

 myzon and Squalus. Somites 1, 2, and 3 of Anamniote emlnyos are represented 

 in birds by the three 'head-cavities' which form the eye muscles. Abbrevia- 

 tions: I-VII., neuromeres 1 to 7; a., anterior entodermic diverticulum (the 

 homologue of the 'anterior' head-cavity of Elasraobranchii and Ganoidei; 

 c.s.g., anlage of the 'club-shaped gland' (first pair of visceral pouches in Am- 

 phioxus); my.i, my.'^, iny.^, . . . my.'', mj^otomes 1 to 7; phy., pharyngeal 

 region; sp., spiracle. 



