308 



Embryogenesis: Progressive Differentiation 



specific morphogenetic and histogenetic po- 

 tencies corresponding to the successive levels 

 of the central nervous system. These ex- 

 periments have all involved transplanting 

 mesoderm along with ectoderm, and so give 

 no information on the relationship of these 

 layers. No experiments have shown so clearly 

 as those of Clarke ('36), cited for the retina, 

 the exact limits and internal differences of 

 these areas; but it seems likely that they also 

 partake of the field character so well illus- 

 trated there. 



The posteriormost end of the neural axis 

 seems, like the foremost, to depend for dif- 

 ferentiation on special conditions not easily 

 reproduced in transplantation or explantation 

 experiments. These conditions evidently dif- 

 fer in various teleosts: in Fundulus, ex- 

 planted blastodiscs do not form tail at all; 

 in Carassius, tail-like bodies may be formed 

 from isolated or partly isolated blastodiscs, 

 as well as from egg fragments where the 

 injury is not defined. In Epiplatys, Oppen- 

 heimer ('38) found that isolated ventral 

 halves will undergo a tail-like morphogene- 

 sis, but without differentiating any true neu- 

 ral or mesodermal axis, until closure of the 

 blastopore. Removed after this event, an 

 isolate will form a histologically normal 

 tail. This suggests of course that the dorsal 

 lip material is the differential factor. 



In the chick, the situation bears certain 

 resemblances. The posterior levels of the 

 medullary region in primitive streak and 

 head-process stages show considerable un- 

 willingness to differentiate neural tissue un- 

 der various experimental conditions (Wadd- 

 ington, '35; Rudnick, '38a), but may 

 occasionally do so. This differentiation, evi- 

 dently difficult to realize in primitive streak 

 stages, becomes more regular and normal as 

 corresponding parts of older blastoderms are 

 isolated, after the midline axis is brought 

 into place and involution of mesoderm more 

 nearly completed. Spratt's ('52) most recent 

 study on the localization of neural plate 

 gives a clear explanation of these differences 

 on the basis of morphogenetic movements. 

 Small neural masses are regularly formed 

 from appropriate regions of the sinus rhom- 

 boidalis (Seevers, '32; Rudnick, '45). Regu- 

 lation may occur after removal of tail-form- 

 ing material from the sinus; in the tail-bud 

 stage some of the blastema must be left if 

 regulation is to occur (Zwilling, '42). The 

 peculiar mechanics of neural tube formation 

 in the tail region has been pointed out many 

 times; the induction pattern prevailing there 

 remains imanalyzed. 



In addition to the primary neural induc- 

 tion, it is usually believed that the pattern 

 of embryonic organs that soon emerges from 

 the newly-formed germ layers is also a re- 

 sult of induction: that is to say, of contact- 

 or proximity-effect of one layer on another, 

 or of one region on adjacent ones of the 

 same germ layer. No clear formulation of 

 these beliefs has yet been made, or seems 

 possible at this stage of our knowledge. 

 Individual cases, such as the classic eyecup- 

 lens relation, seem in the chick to follow 

 the pattern found in some Amphibia, where 

 the lens may be induced from ectoderm of 

 other body regions by a structure (the 

 future iris border) which itself retains some 

 capacity for lens differentiation (Alexander, 

 '37). McKeehan ('51) has made a stvidy of 

 the fine scale orientation and even fusion 

 of cells during this induction, which may 

 have much wider application than to this 

 one case. The chick otocyst (Waddington, 

 '37; Levi-Montalcini, '46) evidently depends 

 for its origin on several regional factors, 

 rather than on any one tissue. In the fish, 

 a wide variety of embryonic regions may be 

 induced to form ear (Oppenheimer, '38) 

 when transplanted to the embryonic shield — 

 not necessarily in a region corresponding 

 to the host ear. In Salmo, Eakin ('39) found 

 a considerable extent of the archenteron roof 

 capable of inducing (directly or indirectly) 

 otic vesicles in extraembryonic ectoderm. 

 Luther ('36a) found ears in grafts from 

 various sectors of the gastrula, approxi- 

 mately corresponding to the distribution of 

 nervous tissue, although less frequent in 

 occurrence. 



In the chick, the prospective nasal placode 

 appears to depend for differentiation upon 

 both brain and mesoderm, in early stages 

 (Street, '37). The origin and dependence of 

 other extramedullary contributions to the 

 nervous system — various placodes, neural 

 crest — have not been studied. The epidermis, 

 too, has been so little investigated that it 

 is perhaps well to state here that it is not 

 a negative tissue, the product of differentia- 

 tion by default of anv ectoderm not receiving 

 another induction stimulus. The epithelium 

 found in hyperblastulae of teleost eggs, or 

 in chick blastoderms in which the axis has 

 been suppressed, is not epidermis; it more 

 resembles the epithelium of extraembryonic 

 membranes. Special relations to dermatome 

 and other mesoderm, and to neural crest, 

 must be factors in producing the differenti- 

 ated tissue and its derivatives. Th° studies 

 of Saunders ('48) on the remarkable pe- 



