340 A2fNALS XEW YORK ACADEMY OF SCIENCES 



The steps by which the pn'Trn'tive radiate s^Tniuetry changed into bilat- 

 eral symmetry are quite vague, but the change no doubt involved the 

 progressive contractility of the myocoelomic pouches and the assumption 

 by some of them of a partly locomotive function. 



Bilateral symmetry was, at least, one of the earliest of all chordate ac- 

 qtiirements and may have led the way for such fundamental characters 

 as metamerism, a notochord and a nervous system of vertebrate type. 

 The myoccelomic pouches very early began to increase in size and num- 

 ber and in power of contractility; all this accompanying an emphasis of 

 bilaterality, a moderate lengthening of the anteroposterior axis and the 

 assumption of an obtusely fusiform shape. The nervous elements, per- 

 haps originally more or less diffused in the skin, were segregated in defi- 

 nite tracts, which foreshadowed the chordate neuron and segmental 

 nerves. The anterior part of the gut gave off paired diverticula on either 

 side, which may at first have served to draw in a food-bearing current of 

 water, but later assumed a respiratory function, acquired exterior fistulas 

 and gave rise to the branchial apparatus of chordates. The myocoe- 

 lomic sacks, extending ventrally, inclosed the primitive gut below: the 

 coelenterate mouth was closed and a new mouth was opened, formed from 

 the coalescence of opposite pharyngeal diverticula. The dorsal moiet}' of 

 the myoccelomic sacks gave rise to the muscle segments, the ventral moi- 

 ety to the coelom. Meanwhile the mesenchyme, which in modern verte- 

 brates arises chiefly from the splanchnic and somatic walls of the muscle 

 plates, was giving rise not only to the corpuscles of the blood and lymph, 

 but also to the deeper layer of the skin (corium), the involuntary muscles 

 and the connective tissue antecedents of cartilage and bone (Eangslev), 

 all of which were destined to become of the utmost importance in the 

 further development of the locomotive organs. 



The notochord perhaps arose as a ciliated groove on the dorsal wall of 

 the gut, its locomotive function being secondary. The circulatory, excre- 

 tory and respiratory structures were all accessory adaptations for more 

 rapid metabolism, following the primary change of myocoelomic pouches 

 into locomotive organs. During these early stages there was but little 

 differentiation of head and trunk, the phar^-ngeal region was large, and 

 cephalogenesis, or fusion of neuromeres etc., was in a low stage. 



A critical stage in vertebrate evolution was reached when, through the 

 cooperation of the vascular system and of the rapidly differentiating 

 mesenchyme, a twofold skin was substituted for the primitive ectoderm, 

 and connective tissue began to be formed around the notochord, around 

 the sensorv capsules, beneath the skin, between the myomeres and be- 

 tween the gill pouches. Still later the process of ossification and calcifi- 



