-9- 



dlal laniella is formed as far back as the opening of the gland, 

 before the lateral invagination takes plaoe, but in my material 

 this is certainly not the case, as is shown in "ig. 2 b. 



T'he lateral invaginations extend posteriorly to a 

 point a little behind the anterior edge of the slit-like opening 

 into the pharynx, "ig 2 c being from a section that cut through 

 the extreme posterior part of the invagination in the right side 

 but was too far posterior to out that of the left side, either 

 because the section was somewhat oblique or because the right 

 invagination had proceeded further than the left^ 



It is well to get a clear idea of these lateral invag- 

 inations in the simple condition represented in '^ig 2 b, as the 

 more complicated later stages will thus be more easily under- 

 stood ere the lateral walls straightened out, thus obliterat- 

 ing the lateral invaginations, the thyroid would be reduced to 

 a more or less cylindrical body, as it was in "ig lb, though 

 with a much larger cavity he long cells of the lateral walls 

 are divided by the invagination into two groups, which become 

 more and more distinct as development proceeds '''n ^Ig 2 b 

 one of these groups, on each side, lies between the invagination 

 and the perpendicular part of the cavity of the gland; the other 

 group lies betv/een the invagination and the dorsal, horizontal 

 part of the cavity '"ompare "ig 5 e ) '^he cavity of the 

 thyroid between this point and the posterior edge of the ox^ening 

 into the pharynx is simply a deep and narrow groove, slightly 



