Charles Eiissell Barcleen and Warren Harmon Lewis 



31 



Fifth Week. 



During the fifth week of development a considerable amount of 

 organization occurs in the spinal axis, the body-wall and the limbs. The 

 nature of the processes taking place are indicated in Embryo CLXIII 

 (length, 9 mm.; probable age, 4^ weeks). 



The structure of the back, the limbs and body-wall in this embryo is 

 shown in Fig. B, Plate II, and Fig. C, Plate III. The areas mentioned 

 are drawn from reconstructions. The remaining parts of Pig. C are 

 drawn from an excellent photograph. 



Fig. 28. Diagrammatic cross section tlirough the 5th-Gth thoracic segments of 

 Embryo CLXIII. x 25 d. The general arrangement of the structures represented is 

 like that described for Fig. 21. 



Fig. 23 shows diagrammatically the general nature of the structures 

 in a typical thoracic segment (the 6th) of Embryo CLXIII. The 

 changes taking place in the thoracic region during the first half of the 

 5th week may be readily followed by comparing Fig. A with Fig. B, 

 and Fig. 21 with Fig. 23. 



The skeletal portion of the segment consists in Embryo CLXIII, as in 

 Embryo II, of a condensed mesenchyme at the distal third of the seg- 

 ment, but the scleromere is far more definitely outlined. Xeural and 

 costal processes are well developed. The latter present something of 

 the general form of ribs in the thoracic region (see Fig. 23 and Fig. B). 

 A sheet of condensed mesenchyme connects the neural and costal process 

 of the scleromeres of neighboring segments. That connecting the 



