2l8 



EARLY STAGES OF ARTHROPOD AND VERTEBRATE EMBRYOS. 



formed at successive periods, the oldest and most highly specialized around the 

 central infolding, the newest and least specialized on the periphery. 



The bilateral type appears to have arisen from the radiate by a local 

 outgrowth from it, not by a transformation of its entire body. The outgrowth 

 gave rise to the new body, and the old body became the head of the new animal. 



A premonitory stage of this transformation may be seen in the one tentacled 

 ccelenterate larvae, as I pointed out in 1889, and the same type may be again seen 

 in the trochosphere, and in the early embryonic stages of all segmented animals 

 We may represent such forms, laid down in mercator projection, by a racquet- 

 shaped plate, the large anterior end representing the body of the ccelenterate 

 that is to become the head, the handle representing the outgrowth from it that is 

 to form the new body. (Fig. 120.) 



Figs. 119— 120-121. — Diagrams, in mercator projection, illustrating the three principal types of growth, 

 and the coincident morphological and physiological differentiation. Fig. 119. — Radial type, showing the lines 

 of unequal physical and chemical stress created by radial growth, and their coincidence with the lines of 

 morphological differentiation. Fig. 120. — Apical bilateral type, showing the origin of bilateral symmetry, 

 apical growth, and metamerism, as a result of unequal radial growth. Fig. 121 . — Apical asymmetrical type, 

 or false radial type, derived from the apical bilateral type, by the suppression of growth on one side. 



Whatever may be the cause of the unequal radial growth, once established, 

 it automatically creates bilateral symmetry and metamerism. To illustrate: If 

 in a growing circular disc, there is, for any reason, a local increase of radial growth, 

 the resulting form will be oval, or triangular, or banjo-shaped. (Fig. 120.) 

 The isogeminal lines will then form two similar, converging series on either side 

 of the enlarged radius. At any time in the history of this organism, there will 

 be a graded linear series of cells, from the oldest at a, to the youngest at an, and a 

 double graded series from any point in a-an to the right, or left. 



There can be but two points in the entire mass at any time that are alike 

 as to age, an environment, each one lying in a corresponding position to the other 

 on opposite sides of the principal axis of growth. As a result of apical growth, 



