96 



CLEAVAGE AND DIFFERENTIATION 



eggs involves a decrease in the activities of the cytoplasm, or, in 

 other words, a relative increase in the inertia of the mass of yolk. 

 At all events, the splitting into two of the blastopore lip has been 

 observed in such eggs, at the onset of gastrulation.^ An additional 

 result obtained in these experiments is cases of disorganised growth, 

 leading to tumour-like proliferations, which increase at the expense 

 of the embryo itself, may give rise to metastases, and can be 

 propagated by grafting (fig. 43). 



Fig. 43 

 The effect of delayed fertilisation in frogs' eggs. Duplication, teratological 

 monstrosities, and tumour-like growths in tadpoles derived from late-fertilised 

 eggs (over 3 days over-ripe). Top left, anterior duplication, the upper head im- 

 perfect , with single sucker. Top right, tadpole with irregular tail and rudimentary 

 secondary ("parasite") head. Below, larva with much reduced head and 

 tumorous growths ventrally. (Redrawn after Witschi, Verh. Naturfursch. Ges. 

 Basel, XXXIV, 1922.) 



§3 



Turning now to the experiments of isolating blastomeres in other 

 groups of animals, it was found that the results differ considerably 

 in the various groups. In some forms, the isolated blastomere de- 

 velops into a complete and normally proportioned larva, differing 

 from a normal larva merely in its small absolute size. In other 

 forms, the isolated blastomere is incapable of doing this, and gives 

 rise to a partial structure only. As extreme examples of these two 

 types we may take the Hydrozoa and the Ascidians, respectively. 



^ Witschi, 1922, 1930. 



