88 LEONARD W. WILLIAMS 



the longitudinal ectodermal invagination which marks the for- 

 mer lateral boundary of the somite. The center of the cutis 

 plate, comprising about one-third of the whole, is now entirely 

 transformed into dermal mesenchyma, and the adjacent portion 

 of the cutis plate, including perhaps another third, is partially 

 transformed into mesenchyma. The dermal mesenchyma of 

 this somite is quite dense. It extends from the myotome to the 

 ectoderm and it also projects in the form of a thin sheet between 

 the partly transformed area of the cutis plate and the ectoderm. 

 Ventrally the dermal mesenchyma reaches nearly to the upper 

 edge of the mesenchyma of the Wolffian ridge, from which, how- 

 ever, it is separated by a small space that intervenes between the 

 ectodermal ridge and invagination that mark the former lateral 

 boundary of the somite and of the dermomyotome. The presence 

 of this gap between the mesenchyma of the cutis plate and that 

 of the parietal plate makes it clear that the former does not arise 

 from the latter but that the two mesenchyma masses arise sepa- 

 rately. The later development of the myotomes of the trunk has 

 been followed by Engert. 



THE FORTY-FOURTH SOMITE 



The history of the forty-fourth segment does not differ much 

 from that of the twenty-fifth segment except that shortly after 

 its differentiation into muscle, and dermal and axial connective 

 tissue, it degenerates. In an embryo of forty-nine segments 

 (H. E. C'. no. 478) the core of each somite of this segment has 

 fused with the lower part of its cortex and both the upper and the 

 lower myotomic grooves are present. The upper groove is more 

 distinct than in the twenty-fifth segment, and the lower groove 

 is deeper on the lateral than on the medial surface of each somite. 



The dermomyotomes of the forty-fourth segment of an embryo 

 of fifty-one segments (H. E. C. no. 526, 5 days), are complete, 

 but are not cut off from the sclerotomes. The sclerotomes of 

 this segment have fused with the large mass of hypomerous 

 mesenchyma and with the adjacent sclerotomes. 



