S68 PATTERNS AND PROBLEMS OF DEVELOPMENT 



tial, stomodeum. That the extra entodermal pouch is always the result of 

 mechanical division of the entoderm by the stomodeum, rather than a true 

 reconstitution, may be questioned. And, finally, reconstitution in early 

 stages may be Hmited by viscosity of the cytoplasm. The fact that forms 

 developing from isolated blastomeres remain flattened on the side former- 

 ly in contact with other cells indicates a considerable physical rigidity. 

 Spek finds a marked increase in viscosity at the third cleavage. Physical 

 rigidity, as well as regional specificity, may limit reconstitution. 



Evidently the undivided ctenophore egg is not a mosaic but a dynamic 

 system in which definite orderly and related changes take place. From the 

 data at hand it appears possible that before polar-body formation the 

 cytoplasm has no definite organization except a surface-interior difference 

 and that axiate pattern develops gradually. The ectoplasmic activities of 

 first and second cleavage may be factors in determining the biradial pat- 

 tern. In short, it seems possible that the polar-biradial pattern of the cten- 

 ophore can originate without any further "organization" than the surface- 

 interior, ecto-entoplasmic pattern, the eccentric position of the nucleus, 

 and the ectoplasmic activities during the first and second cleavages. Since 

 the plate row of the adult and the general ectoderm show a very definite 

 apicobasal gradient (p. io6) and the apical region is dominant in recon- 

 stitution (Coonfield, 1936a), gradient pattern must develop in the de- 

 scendants of the micromeres at some stage. 



CLEAVAGE PATTERN AND DEVELOPMENT OF AscaHs 



Cleavage pattern, cell hneage, and the "germ path" of Ascaris megalo- 

 cephala {^equorum) have been repeatedly and extensively studied.'^ The 

 germ path, that is, the persistence of a small number of large chromosomes 

 in the blastomere fine from which the germ cells develop, and the diminu- 

 tion of chromatin by exclusion from the nucleus of the terminal portions 

 of the large chromosomes and appearance of a larger number of small chro- 

 mosomes in other cells, has been regarded as a feature of particular inter- 

 est (Fig. 178). Essentially similar cleavage patterns have been observed 

 in various other nematodes, but chromatin diminution apparently does 

 not occur in all. 



The egg and embryonic stages of Ascaris, inclosed, as they are, in a 

 hard thick shell, are less directly accessible to some experimental pro- 

 cedures than those of many other forms; but blastomeres have been killed 



'^ Boveri, 1887, 1890, 1892, 1899, 19106; C. C. Schneider, 1891; Herla, 1893; Zur Strassen, 

 1896; Zoja, 1896; H. Miiller, 1903; Bonfig, 1925; et al. 



