658 PATTERNS AND PROBLEMS OF DEVELOPMENT 



ISOLATED DEVELOPMENT 



Oocytes of certain annelids separate from the ovary at an early stage 

 and pass their whole growth period as free cells in the coelomic fluid. The 

 oocyte of Arenicola crislata, an example of this type, is, when full grown, 

 flattened to a shape approaching a biconvex lens, the axis of flattening 

 apparently becoming the polar axis. The egg also differs somewhat in 

 diameter in two directions at right angles to each other and in a plane 

 vertical to the short axis, and these two diameters are probably parallel 

 to the first two cleavage planes. There is no visible regional differentia- 

 tion, not even localization of yolk.^ After separation from the ovaries 

 the environment of these oocytes is presumably not differential in any 

 definite or persistent way, since the waves of peristaltic contraction of the 

 body wall keep them almost continuously in motion. Nothing is known 

 of origin or nature of the primary developmental pattern of this egg. 

 For those who beheve that spatial organismic pattern is an inherent prop- 

 erty of a species-protoplasm or that it can originate in a single isolated 

 cell without any relation to external factors, the Arenicola egg provides 

 an apparent example of such origin. However, the possibility remains 

 that the basis of pattern may be established before isolation from the 

 ovary and that it persists because the uniform environment of the free 

 cell provides no differential to alter it. 



IMBEDDED DEVELOPMENT 



Coelenterates and sponges present examples of this sort of relation 

 between oocyte and body, the oocyte developing as a naked cell in the 

 tissues of the parent body and evidently obtaining nutrition from sur- 

 rounding cells or from intercellular fluids. These oocytes are often amoe- 

 boid in earlier stages and may migrate. In the hydrozoa, oocyte develop- 

 ment takes place in the ectoderm; in scyphozoa and anthozoa, in the 

 entoderm; and in sponges, in the mesogloea. Usually it is more or less 

 locahzed in certain regions of the body — for example, on the radial canals 

 or the manubrium in hydromedusae, on the mesenterial borders in an- 

 thozoa — but an ovary, as a special, definitely localized and bounded or- 



development, that is, without special nutritive or other accessory cells, and alimentary develop- 

 ment, with relation to nutritive cells; when nutritive cells surround the oocyte development is 

 follicular, when they are single or several in a group it is ntitrimentary. The present considera- 

 tion, being primarily concerned with the question of the possible role of conditions of oocyte 

 development in determining pattern, employs a somewhat different classification of develop- 

 mental types. 



' Child, 1900; A. marina, Child, 1898. 



