38 PATTERNS AND PROBLEMS OF DEVELOPMENT 



Morgan (igoib, 1903c), and Child (igo'jb-e) have shown that length of 

 hydranth primordium differs in definite ways at different levels of section 

 on oral and aboral ends of the same piece and in short pieces with differ- 

 ence in length of piece. These differences involve differences in localiza- 

 tion and length of parts of the hydranth, that is, differences in scale of or- 

 ganization. Some of the measurements are given in order to show what 

 actual lengths are found and how they differ. Driesch (18996), using two 

 successive pieces of T. mesemhryanthemiim, each 7-10 mm. in length, gave 

 the following measurements: oral primordia of distal pieces, 1.67 mm., 

 of proximal pieces, 1.21 mm.; aboral primordia of distal pieces, 0.96 mm., 

 of proximal pieces, 0.9 mm. Measurements by Child (19076), using two 

 successive pieces 20-25 mm. in length, were: oral primordia of distal 

 pieces, 2.26 mm., of proximal pieces, 1.33 mm. With three successive 

 pieces, each 10 mm. in length, primordium lengths were as follows: oral 

 primordia of distal pieces, 2.27 mm., of middle, 1.5 mm., of proximal, 1.3 

 mm. Aboral primordia of the same pieces were, respectively, 0.96, 0.85, 

 and 0.91 mm. in length. The slight increase in length at the aboral end of 

 the proximal piece probably results from some degree of physiological iso- 

 lation of this region. With decrease in length of piece, length of oral and 

 aboral primordia becomes, in general, more nearly equal; but even in 

 pieces little longer than the two primordia the aboral may be shorter or 

 less complete proximally than the oral. The proportions of the primordi- 

 um, that is, the relative lengths of the four distinguishable zones (Fig. 13), 

 also show a graded change from distal to proximal stem-levels, and the re- 

 lation of primordium length to length of hydranth after emergence dif- 

 fers in the same way (Child, 19076, c). 



Differences in rate of development and length of primordium similar 

 to those in Tuhularia occur in Corymorpha, another gymnoblast hydroid, 

 with difference in level of reconstitution (Child and Hyman, 1926). The 

 hydranth of this species, however, develops gradually on the naked stem; 

 and even during reconstitution contraction and extension occur; conse- 

 quently, rate of reconstitution and length of hydranth primordium cannot 

 be determined as exactly as in Tubularia. 



The gradient pattern in a branching hydroid is very beautifully shown 

 by Pennaria cavolinii, not only in the growth form but in the order of de- 

 velopment of hydranths after their removal and the order of subsequent 

 degeneration (Cast and Godlewski, 1903). Lateral branches in this species 

 are oblique to the main axis and show a regular gradation in length basip- 

 etally. When all hydranths arc removed, new hydranths develop basip- 



