8o THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS 



in the Ctenophores certain structures are missing from 

 the larvae which develop from isolated blastomeres. 

 The eggs of Nematodes and Ascidians are of the mosaic 

 type, and isolated blastomeres will not develop properly ; 

 one member of the latter group, Styela, may be regarded 

 as an extreme case ; the egg contains a yellow substance 

 which is necessary for the formation of muscle-fibres, and 

 if this substance is redistributed by centrifuging the 

 development is upset. 



The case of the Amphibian egg, which has been much 

 studied, is interesting. Diametrically opposite the point 

 where the spermatozoon enters there forms a '* grey 

 crescent " which eventually gives rise to the dorsal lip 

 of the blastopore (see Fig. 39). This grey crescent is 

 usually divided by the first cleavage plane, and in this 

 case the blastomeres, if isolated, can give rise to whole 

 embryos ; but if the cleavage-plane falls elsewhere, the 

 blastomere which contains none of the material of the 

 grey crescent will not develop. It is possible also to 

 prevent or delay the development of one blastomere. 

 at the two-cell stage, by injuring it with a hot needle ; 

 the uninjured blastomere then behaves as if the sister-cell 

 in contact with it were segmenting normally, and develops 

 *' mosaically " into a half-embryo. Again, it may be possible 

 to effect a partial separation of the two first blastomeres, 

 for example by gentle shaking in the case of the egg of 

 Amphioxus (Wilson) ; this may give rise to " Siamese- 

 twin " or two-headed " Janus " embryos. 



The normal distribution of the nuclei in segmentation, 

 e.g. in the frog or sea-urchin, may be completely upset by 

 gentle pressure, yet normal embryos are produced ; this 

 proves that during cleavage the nuclei of the blastomeres 

 are identical, as far as their efl^ect on differentiation is 

 concerned. 



Variations in the chemical composition of the medium 

 may greatly affect the development of eggs. For example, 

 the fish Fundulus in a solution containing magnesium 

 chloride develops a single median " cyclopic " eye instead 

 of the usual pair of eyes. If calcium, is absent from the 

 medium, the blastomeres of t\\t sea-urchin's egg will not 

 stick together, but become separated. In solutions 



