Teleosts and Birds 



311 



terns, if we compare definitive streak stages 

 for entoderm and heart (Fig. 119E, G, I; Fig. 

 118Z)) with the corresponding head-process 

 diagrams (Fig. 119F, H, J; Fig. 118£), are un- 

 derstandable on the basis of migration toward 

 definitive positions: thyroid anteriorly, liver 

 laterally, intestine posteriorly. The lateral 

 movement of median material and a sharp 

 anterior stretching of material just in front of 

 the node correspond with known germinal 

 movements. In these cases, at least, the hy- 

 pothesis of migration of definitely localized 

 cell groups in mesoderm and entoderm, to- 

 gether, is tenable and preferable. 



Of recent years there has been a tendency 

 to focus attention on the heart area not only 

 as an early autonomous region, present at 

 least as early as localized dorsal axial ma- 

 terial, bvit also as possibly playing an or- 

 ganizing role in the anteroventral region 

 of the body analogous to that of the dorsal 

 lip for the dorsal axis. Direct evidence for 

 this view is lacking; the considerations are 

 early autonomy of the heart, and its tend- 

 ency to be associated with foregut struc- 

 tiH-es (pharynx, thyroid, liver) in experi- 

 mental situations. 



In the case of the intestine (Fig. 119£, F), 

 no cell migrations have been described that 

 will account for the posterior displacement 

 of the area during the head-process stage. 

 It is only fair to say that they have not been 

 looked for, and that the mesodermal portion 

 of the streak would be the place to look. 

 There is evidence that in gut formation, 

 mesoderm plays a large role (Hunt, '37) and 

 tnat hypoblast is, to say the least, dispen- 

 sable in the process. It would be very con- 

 venient if such slight but definite posterior 

 displacement of mesoderm in the streak could 

 be found, since it would then explain very 

 well the posterior expansion of the meso- 

 nephric area between the definitive primitive 

 streak and head-process stages, indicated in 

 Figure 118F, G; this material might be con- 

 sidered to undergo a spurt of posterolateral 

 migration at this time, and to push the lateral 

 plate ahead of it, posterior and lateral to 

 the node field. 



CONCLUDING REMARKS 



The meroblastic vertebrate eggs, by their 

 pattern of differentiation within the ovary, 

 have realized an almost complete separation 

 of the factors and materials involved in pri- 

 mary differentiation, from those necessary 

 for later growth. Thus most teleost blasto- 

 discs can very early be separated from the 



yolk and complete most of the histogenetic 

 and embryogenetic sequence in a saline 

 medium. The trout and chick blastodiscs 

 differ in requiring at least some carbohy- 

 drate from outside (Spratt, '49; Devillers, 

 '49) in order to carry through a comparable 

 sequence of events. It is doubtful if these 

 isolates grow to any appreciable degree, if 

 a rigorous definition of growth as increase 

 in mass be insisted on; by contrast, differen- 

 tiation is remarkably complete. Whether this 

 primary semi-independent phase is also rep- 

 resented in some form in holoblastic eggs, 

 and whether a key to developmental kinetics 

 may be seen in this feature and its variants, 

 is not certain. 



Within the protoplasmic blastodisc of both 

 fish and bird, it seems safe to say that any 

 differentials present during cleavage are 

 fairly easily reversed or negated. That dif- 

 ferentials exist in the fish egg would be 

 indicated by some varieties of experiment 

 where injury or separation results in a de- 

 fective embryo; to balance these, there are 

 clearcut cases of totipotence of halves or 

 even quarters of the blastodisc, continuing 

 up to the blastula stage. Parts of the whole 

 can, when suitably isolated, become wholes 

 themselves. Whether every part of the germ 

 shares this property is still debatable. 



At some time, during or after the onset 

 of gastrulation, some fairly stable differ- 

 entials appear. Thus in Fundulus the dorsal 

 lip region, as soon as identifiable, has po- 

 tencies sharply different from those of the 

 rest of the circumference of the blastodisc; 

 this situation appears to be achieved only 

 gradually in Salmo. Similarly, the duck egg 

 at laying is evidently largely equipotent, 

 whereas all our information on the unin- 

 cubated chick egg points to the posterior or 

 blastoporal sector as having properties not 

 shared by other regions. Thus within each 

 group are found variations of timing. 



Between the two groups, bird and fish, is 

 the major and striking difference of spatial 

 pattern: the position of the blastopore, the 

 relative rate of its formation, the continuity 

 of invagination, and complexities of move- 

 ment in the blastoporal area itself. We can 

 contrast the elaborate history of the primitive 

 streak, with its posteroanterior order of ap- 

 pearance, its delayed invagination and emi- 

 gration, and its complex incompletely under- 

 stood pattern of cellular movement, with 

 what seems to be a much more simple and 

 direct invagination pattern in the teleost. 

 It must be said, however, that great technical 

 difficulties have impeded study of details of 



