Cleavage, Blastulation and Gastrulation 



225 



'27) was that if the late blastula were 

 "water-tight," an absorption of blastocoele 

 fluid by certain cells might cause an in- 

 vagination because of hydrostatic pressure. 

 If gastrulation provides a basis for neuru- 

 lation, the reverse is also true. Glaser ('14), 

 studying the transformation of the neural 

 plate into a neural tube, found that there 

 was no increase in number of cells during 

 the process. Therefore, there is no asym- 

 metrical proliferation of cells to account for 

 neurulation. During the process of folding, 

 however, the volume of the neural plate was 

 found to increase about three-fold by the 

 intake of water. This was assumed to ac- 

 count for the fact that about 80 per cent 

 of the nervous system of an amphibian is 

 made up of water, whereas water consti- 

 tutes only about 60 per cent of the whole 

 body weight. In a later note (Glaser, '16), 

 the lack of evidence for differential water 

 absorption within the neural plate itself is 

 stressed. Brown, Hamburger, and Schmitt 

 ('41) did not confirm Glaser's values for 

 water intake during gastrulation and neuru- 

 lation. They obtained very slight changes 

 in density which were interpreted as show- 

 ing that there are but slight changes in 

 volume or water content of the prospective 

 neural tissvie during development. 



Holtfreter ('43b, '44) attempted to corre- 

 late the movements of the several regions 

 of the amphibian embryo during gastrula- 

 tion with the inherent properties of certain 

 cell groups, and to the responses of these 

 cell groups to stimuli emanating from their 

 environment. The primarj'^ cause of gastrula- 

 tion in the Amphibia, according to his 

 theory, is a higher relative alkalinity of the 

 blastocoele fhiid, which brings about local 

 surface tension changes in the membranes 

 of certain flask-shaped cells. These cells 

 are attached at their outer boundaries to the 

 "surface coat" and exert contractile tension 

 when they elongate in the direction of the 

 blastocoele in response to the surface ten- 

 sion changes at their blastocoelic boundaries. 

 Modern cellular physiologists, however, have 

 all but abandoned surface tension changes 

 at cell surfaces as being of any great sig- 

 nificance, since all actual measurements 

 (E. N. Harvey, '31; Cole, '32; Cole and 

 Michaelis, '32) of the magnitude of the ten- 

 sion at the surface of cells have indicated 

 an almost infinitesimally small value. As 

 indicated above, if the surface tension is low, 

 agents producing slight changes in it can- 

 not produce much of a physiological effect. 

 Lewis ('47) attempts to explain invagina- 



tion and neurulation in terms of one essen- 

 tial factor. This is an increase or a decrease 

 in the contractile tension of the superficial 

 gel layers on one side of a group of adherent 

 epithelial cells which offer some resistance 

 to distortion. This is a purely mechanical 

 "explanation," since it would be necessary 

 to elucidate the causes of the increase or 

 decrease of contractile tension in relation to 

 the biological variables of time, place, and 

 cell type. 



Spek ('20) used as a model of gastrulation 

 a composite strip of two layers of gelatin 

 with different densities. When immersed in 

 water, this composite strip absorbed water 

 differentially and curved to simulate the 

 behavior of a portion of the entodermal plate 

 of the invaginating gastrula. Spek also sug- 

 gested that the lithium in Herbst's experi- 

 ments might make the outer portion of the 

 blastula wall lyophilic, as compared with 

 the inner wall, and thus bring about 

 evagination. Moore ('30) has pointed out 

 that in Herbst's original work, it was dem- 

 onstrated that the characteristic effect of 

 lithium is less if applied after the 16-cell 

 stage. The fact that lithium is not effective 

 in causing evagination if applied to the early 

 blastulae indicates that the cells have been 

 determined in their invaginative pattern at 

 an earlier stage. The Spek hypothesis of the 

 mechanism of lithium action in terms of a 

 direct action to produce lyophilic colloids at 

 a time immediately preceding gastrulation 

 is therefore inadequate. , 



Moore ('30) tested Assheton's hypothesis 

 by experimentally producing various geo- 

 metrical relations of nuclei to cells m late 

 blastulae of the sand dollar and sea urchin. 

 The position of the nuclei with respect to 

 the tendency of regions to evaginate or in- 

 vaginate showed little or no correlation m 

 this experimental material. 



In more recent contributions, Moore and 

 Burt ('39) and Moore ('41) attempted to 

 analyze by operative means the question 

 whether the entire blastula takes part in 

 the gastrulation process, or whether the 

 forces of gastrulation are locahzed m a spe- 

 cial region. When the upper half or two- 

 thirds of the early gastrula is cut away, 

 leaving an intact vegetal or entodermal 

 plate, gastrulation is not impaired 1 his ex- 

 periment disposes of the idea that differences 

 between the blastocoele fluid and the ex- 

 ternal medium play a part during invagina- 

 tion In the cases where the entodermal plate 

 was isolated by cutting away the remainder 

 of the embryo, the plate continued to roll 



