90 LEONARD AV. WILLIAMS 



A given somite does not recapitulate the development of each or 

 any preceding somite; on the contrary, each succeeding somite 

 is in a more advanced structural condition at the time of its 

 formation than the preceding somites. 



Each somite divides first into the primary sclerotome, formed 

 by the fusion of the core with the floor and the lower part of the 

 walls of the somite, and the dorsal lamella which consists of the 

 roof or dorsal wall of the somite and a bordering zone of growth 

 developed in the upper part of the medial, anterior, and posterior 

 walls and, in the somites of the trunk, of the lateral wall also. 



The dorsal lamella gives rise to the dermomyotome and also, I 

 believe, to two large dense masses of sclerotomic mesenchyma, the 

 anterior and posterior "halves" of the sclerotome, or, as I perfer 

 to call them, the anterior and posterior sclerotomic columns. 



The myotome, or muscle plate, is formed in a large measure, 

 particularly in the occipital and cervical regions, by proliferation 

 from the medial edge of the dorsal lamella but in the trunk and to 

 a less degree in the tail, the lateral edge of the dorsal lamella 

 contributes to its growth. 



The anterior and posterior edges of the dorsal lamella contrib- 

 ute but slightly to the myotome. They produce, however, a 

 large amount of mesenchyma which is added to the primary sclero- 

 tome and which forms the dense anterior and posterior columns 

 of the secondary sclerotome. 



The sclerotomic columns, which are usually regarded as mor- 

 phological units that represent one or more elements of a primarj^ 

 or secondary vertebra, are actually composite structures in which 

 the several morphological elements cannot be distinguished. 



The perichordal sheath, in which the centra and intercentra 

 arise, is formed by the fusion of the notochordal processes of the 

 primary sclerotomes. The notochordal processes of the first 

 fourteen pairs of sclerotomes, exclusive of those of the first seg- 

 ment, grow far forward under the preceding segments; conse- 

 quently parts of the perichordal sheath can be only vaguely as- 

 signed to particular segments. Each centrum arises in an inter- 

 segmental part of the perichordal sheath and is formed from loose 

 tissue ; each intercentrum, on the contrary, arises in a midsegmen- 



