176 BULLETIN OF THE 



but one cell thick, hence I am warranted in assuming that a certain 

 thickness pertains to the mesoderm from which a proto vertebra is im- 

 mediately to be formed. Since the breaks iu the mesoderm appear so 

 gradually, it is possible, at the time the first cleft has appeared, to see 

 where the next is to follow, and consequently to determine definitely 

 on which side of the first protovertebra the first cleft appeared. 



Figure 1, Plate I., is a section through a chick at the time when one 

 decided parting of the mesoderm occurs, and the chick when examined 

 entire by transmitted light showed but one mesodermic cleft. 



From this section it will be seen that the first cleft lies anterior to 

 the first protovertebra, not posterior, as Kupffer and Benecke supposed. 

 A partial cut separates this protovertebra from the posterior mesoderm, 

 while the protovertebra anterior to the first cleft is still closely con- 

 nected with the mesodermic band which runs forward to the head. In 

 fact, the anterior protovertebra is not entirely separated from the 

 anterior mesoderm until as many as four or five protovertebrse have 

 been formed. A curve in the dorsal outline of the mesoderm indicates 

 from the first, however, where the anterior protovertebra is to be ulti- 

 mately cut off. The series from which Figure 1 is taken shows more 

 conclusively than any other series which I have, the relation of the first 

 mesodermic cleft to the first protovertebra. It is difficult to find the 

 exact stage when the first cleft is far more developed than the second, 

 although the second one is already distinctly marked. However, in 

 almost any series made at the time when the first protovertebra has 

 just been formed, the same conclusion with regard to the priority of 

 the anterior cleft may be reached by comparing the number of entire 

 and partial breaks which occur in successive sections anterior to the 

 first protovertebra with those that occur posterior to it. It will then 

 be found that the anterior cleft is both deeper and more continu- 

 ous than the posterior. The sections which I have drawn have 

 been selected from chicks, the stages of whose development follow 

 so closely upon one another, that, if it be supposed that between 

 any two stages the protovertebra marked a has developed into that 

 marked 1, while the cells included between the letters / and g have 

 developed into the protovertebra a, then the protovertebra?, which 

 in the preceding section were already partly cut off, posterior to 1, 

 must have become attached to the mesoderm from which they were 

 thus partly separated, or the order of development must vary in dif- 

 ferent chicks ; for if the above supposition were true, in the chick with 

 the greater number of protovertebra? the posterior ones would be less 



