A DESCRIPTIVE SKETCH 



29 



folds. But it is to be noticed that these caudal muscles arise from 

 material that has never been invaginated. 



This state of affairs need not give rise to undue astonishment, 

 for the region from which this presumptive caudal muscle material 



Fig. 9 

 Four stages of development of an embryo of Afublystoma fnexicanum to which 

 intra vitam stain marks were appHed as shown in a ; mark 1-2 is later found in the 

 epidermis of the mid-dorsal line and in the hinder part of the neural tube ; mark 

 2-3 in the epidermis of the tip of the tail and in the hinder muscles of the tail; 

 mark 3-4 in the epidermis of the mid-ventral line and in the muscles of the base 

 of the tail and the hinder part of the trunk. Mark 3-4 has been invaginated in 

 part; the other marks have not been invaginated, but mesodermal muscles have 

 nevertheless been formed from the median part of mark 2-3. (From Bijtel, 

 Arch. Entzvmech. cxxv, 1931.) 



arises lies immediately to the side of the blastopore at the moment 

 of the latter's closure, and at the blastula stage it lay touching the 

 presumptive regions of the hindmost mesodermal somites of the 

 trunk. It might be said that if the blastopore did not close so soon 

 but remained open for a little time longer, it would tuck in and 



