42 bulletin: museum of comparative zoology. 



The loose tissue shown at va. sng. in Figure 3 has in this stage 

 developed into the walls of blood spaces (Plate 2, Figure 13, va. S7ig), 

 which as yet contain no corpuscles. 



Stage IV, 



This stage may be considered rather briefly. The embryos measure 

 from G to 6.5 mm. in length. The deeper layer of the ectoderm over 

 the optic vesicle has invaginated to form a conical lens-thickening of 

 high columnar cells. 



The cells of the splanchnic layer of the epimer have increased in num- 

 ber and are beginning to take on the form of muscle cells, but there is 

 no indication of fibrillae. 



Figure 2 (Plate 1) shows the main features to which I wish to call 

 attention. These are the further migration of the duct {dt. Wf.) inward 

 under the somite, and the growth of the sclerotome (scVtm.), which has 

 now extended dorsad between epimer and neural tube and encountered 

 the sclerotome of the opposite side of the body. Loose cells of unknown 

 origin lie where the aorta later appears and presumably contribute to its 

 formation. Similar scattered cells are commonly found beneath the 

 whole of the ventral wall of the somite, where they later become much 

 more numerous. In sections posterior to the one figured, the lumen of 

 the somite {c.oel. so.) extends up to its dorsal angle. 



Stage V. 



I take as a type of this stage an embryo 9.25 mm. in length. A con- 

 siderable advance over the last stage has been accomplished in the 

 specialization of tissues. The main mass of the epimer is composed of 

 longitudinally directed muscle cells whose nuclei occupy a central posi- 

 tion. In frontal sections the non-nucleated ends of the cells are seen 

 to form broad, clear l)ands at either end of the somite. The clear bands 

 of successive somites are separated by a thin layer of connective tissue, 

 the myocomma (Plate 1, Fig. 7, my'cm., and Fig. 9). Muscle fibrillae 

 are clearly distinguishable. The sclerotome has only occasionally the 

 form of a continuous layer of cells, having been for the most part 

 transformed into finely branching, mesenchymatous tissue with scattered 

 nuclei, which fills the space between the somites, the neural tube, 

 chorda, etc. Between the ventro-median side of the somite and the 

 entoderm the sclerotome seems to retain some of its continuity with the 

 somite, but how much of that tissue (Fig. 8, y8) is sclerotome and how 

 much has arisen from the loose cells mentioned in the description of 



