Embryonic Growth in Animals 209 



The problem of the determination of strophosomy has been fur- 

 ther studied by local applications of colchicine in agar strips.^^^ In 

 embryos with 25-28 somites, the region between the omphalomesen- 

 teric vessels and the hind limb is the most sensitive in regard to this 

 malformation. Absence of tail and hypophalangism and absence of 

 tail were also observed; these phenomena led to a study of colchicine 

 on the expression of the anomaly, polydactyly.^'^ In other animals, 

 colchicine is also a teratogenic agent,* but the changes mentioned are 

 of very different types, ranging from exogastrulation" to variations in 

 pigmentation, cyclopean eyes, abnormal blood formation, and dis- 

 turbances of body flexures.^'* In the frog, many of the reported 

 anomalies^-- ^'^ could also be initiated by X-rays, a fact strongly 

 suggesting their relation to mitotic disturbances. 



One other result is worth mentioning. Local application of a 

 1:7000 solution of colchicine on the posterior limb of Xenopus larvae 

 resulted in a decrease in the number of toes.^ With increasing effects 

 all but the fourth toe disappeared during development. This is 

 paralleled by no other type of regressive evolution of toes in verte- 

 brates. 



8.3: A Tool for the Study of Embryonic Growth 



The use of colchicine for the detection of zones of maximal 

 growth and of growth stimulation or inhibition will be discussed at 

 length in Chapter 9. The "colchicine method" is fundamentally 

 based on the observed increase in metaphases, arrested because of 

 the absence of spindle, in growing tissues. Mitotic multiplication of 

 cells is made more visible. Some of the difficulties of this method in 

 adult animals will be discussed in Chapter 9. It is evident from all 

 that has been written in this chapter, that in embryonic growth the 

 complexity of the changes brought about by colchicine is consider- 

 able. Not only does the alkaloid inhibit mitoses, it may also com- 

 pletely alter the normal course of growth. Only a few experiments 

 yield facts that are simple to interpret. 



For instance, in chick embryos treated at the forty-second hour 

 of development with dilute solutions of colchicine, there could be 

 observed, 24 hours later, an "overjiroduction of cells." -'^ The amount 

 of neural tissue appeared to be increased, and several neural folds 

 were to be seen, even in animals where the number of arrested mitoses 

 did not appear to be great. These facts were considered as good 

 evidence of mitotic stimulation and increased neuralization, that is 

 to say, a colchicine-induced malformation. Chicks with spina bifida 

 have been found in some experiments.^ The number of mitoses 

 seemed considerable to the author who observed for the first time 

 these neural changes, but no accurate quantitative counting w^as done, 

 nor, in fact, could have been properly done because of the malforma- 



