DEVELOPMENT OF THE INDIVIDUAL 



575 



tion of the vertebrate eye. As the bulb of 

 tissue that pushes out from the brain to form 

 the retina comes near the ectoderm it in- 

 duces the latter to form a lens. This will 

 happen even if foreign ectoderm is placed 

 over the eye cup, in other words, whenever 

 the eye cup approaches ectoderm, it induces 

 a lens in that germ layer. In both of these 

 cases it is apparent that something, most 

 likely a chemical, issues from the organizer 

 (dorsal lip cells and eye cup, respectively) 

 and induces the formation of the second 

 structure. 



These observations pose an important 

 question. Is embryonic development due to 

 these organizers, which are produced, per- 

 haps, one after the other by different tis- 

 sues? Does one organ develop as a result 

 of one organizer and does that organ, in 

 turn, produce another organizer that stimu- 

 lates the development of another organ and 

 so on? This may be true, but until more is 

 known about what an org;anizer is, the an- 

 swer to this question will be slow in coming. 



Regeneration 



In earlier chapters we considered tlie 

 phenomenon of regeneration among the in- 

 vertebrates, especially in hydra and pla- 

 naria where it is particularly striking. Can 

 the observations made on these forms, 

 and others as well, be explained in the 

 light of the information available as a result 

 of experimental embryology? 



Polarity is one of the first features that 

 appears in embryonic development. Can 

 such polarity be demonstrated in a coelen- 

 terate and, if so, can it be controlled? If the 

 stem of a hydroid is cut into two equal 

 parts, each will give rise to a base and ten- 

 tacles, retaining the polarity of the parent 

 form (Fig. 23-14). This is reminiscent of 

 polarity in early embryos. Now if these two 

 parts are placed in a tube and the cut tips 

 exposed to different concentrations of oxy- 

 gen, the polarity can be reversed (Fig. 23- 

 14). This means that organization of the 

 tissue is labile, that is, it can be changed. 



This can be further shown to resemble the 

 condition in the amphibian egg by bringing 

 about a partial constriction in the mid- 

 region of a section of the hydroid ( Fig. 23- 

 14). In this case, tentacles form at both 

 ends. How can this be explained? Accord- 

 ing to Professor L. G. Barth, we might think 

 that perhaps some organizing substance is 

 distributed throughout the tissues of the 

 hydroid which is tentacle-forming, and that 

 tentacles form where the concentration of 

 this substance is the highest. We would ex- 

 pect the concentration of such a substance 

 to be greatest in the head or anterior region 

 and least near the base. In a cut section 

 there would still be a concentration gradi- 

 ent from anterior to posterior and the ten- 

 tacles would still form at the end where the 

 concentration was the highest, thus main- 

 taining its polarity. 



This same idea might explain why pla- 

 naria, when cut into several pieces, gives 

 rise to an equal number of whole animals, 

 each with mouth, digestive tract, nervous 

 system, and all (Fig. 9-7). In planaria the 

 situation is much more complicated than 

 in a coelenterate, because of the greater 

 variety of structures, but the explanation 

 may still hold. 



Another aspect of this problem has come 

 to light through metabolic studies of a num- 

 ber of lower animals including planaria. It 

 has been shown, for example, that the meta- 

 bolic rate, as demonstrated by oxygen con- 

 sumption studies, is greatest at the anterior 

 and least at the posterior end, and, indeed, 

 that there is a definite gradient running in 

 an anterior-posterior direction. This is con- 

 firmed by applying poisons of various sorts 

 to the animals, in which case tlie anterior 

 end is always injured first, the effect gradu- 

 ally diminishing in a posterior direction. 

 This same gradient is observed in cut pieces 

 of planaria, for the anterior part of each 

 piece has a higher metabolic rate than the 

 posterior part. The head always develops at 

 the more active end and the tail at the 

 other. Long ago, this principle was stated 



