Embryonic Growth in Animals 203 



proceed normally, but in many eggs the grooves faded away later. 

 Even when the concentration was only 1:1,000,000 and when some 

 ajijKirently normal embryos grew, abnormal cleavages were visible, 

 and on the third day all the embryos were found dead. It was evi- 

 dent that even when nuclear mitosis proceeded normally, cleavage 

 could be inhibited. Gastrulation was made impossible, the eggs as- 

 suming a meroblastic type of growth. 



It was soon discovered that in Arbacia the sensitivity of the eggs 

 decreased rapidly after fecundation;^ 40 minutes later, from 90 to 

 100 per cent of normal cleavages could be observed. In the sea 

 urchin Paracentrotus, before fecundation, the eggs may live only in 

 a 1:200,000 solution. Later, cleavage is quite abnormal. If colchicine 

 is apjjlied at fecundation, a 1:60,000 solution does no more than dis- 

 tiab gastrulation. A temperature effect was also observed. Inhibition 

 of growth was nearly complete if colchicine had been allowed to act 

 at 25°C., even if the eggs were kept at lower temperatures later. On 

 the contrary, colchicine at 15°C. permitted growth to the morula 

 stage, or, if the eggs were placed at 25°C. after colchicine, as far as 

 the 16-celled stage. This temperature effect was tentatively related 

 to permeability changes. •'^o 



1 he peculiar behavior of egg cells and the first stages of develop- 

 ment of amphibia have been the subject of a thorough analysis, re- 

 lated in many papers of the French author, Sentein.^*, 35 i^[]^q other 

 workers, he founcl that cleavage disturbances were not closely related 

 to mitotic disturbances; precocious cleavage could, in some eggs, lead 

 to anucleate blastomeres. The complexities of the action of colchicine 

 are revealed by the various cytological anomalies described: poly- 

 ploidy, plurinucleation, asymmetrical development, chromatin bridges 

 between nuclei, pycnosis, and pluricentric mitoses. The last were 

 found during recovery and are comparable to the multiple stars de- 

 scribed in Chapter 3. 



The variable reactions during development were analyzed in 

 Tritunis, Pleurodeles, Bnfo, Rcuia, and Anihlysloma.^^ After gastru- 

 lation, typical arrested mitoses of the star type are the rule, Avith 

 clumped chromosomes that are progressively destroyed. In the earlier 

 stages, however, nuclear changes are quite different. Rather concen- 

 trated, 1:500 and 1:1000, solutions of colchicine were used. How- 

 ever, the cytological changes were always delayed, as observed by the 

 other authors mentioned above. ^' i"^' ^^ First of all, cleavage is in- 

 hibited, the nucleus completing its division. The result of this is the 

 frequent observation of binucleate blastomeres. The spindle may be 

 completely destroyed; large, probably j)olyj)loid nuclei are found 

 later. However, the normal niunber of chromosomes is most often 

 maintained because the spindle, even in these high concentrations of 



