71. 



Pig. 36. B.e const ruction of an embrvo about 12 days old. 



Represented witl-i th.e riglit sJiell valve and mantle 

 lobe removed. 

 Fig. 37, Section of an embrj'-o about / liours old. Tlie 



plane of tlie section lies througli tlie blastopore. 

 Pigs. 33, 39 and 40. Successive sections of an e:TTbr70 



• about I J hours old. Some of tne surface 



cells are beginning to assume the character of 



test colls. The group of cells eg, figures 38 



and 39, are coimected with the cells ap, figure 



40, bv cells of the same character. 

 Pig. 41. Median vertical section of an embryo about 15 



hours old. 

 Pig. 42. Median vertical section of an embryo abo\it 22 



hours old. 

 Pig. 43, Median vertical section of an embrjro about 30 



hours old. 

 Pig. 44. Median vertical section of an embryo aboxit 36 



hours old. 

 Pig. 45, Transverse section of an embryo about 35 hours 



old, taken near the third band of cilia. 

 Fig. 46, Transverse section of an embryo about 45 hours 



old, taken through, the cerebral pouches. 



