The development of the urinogenital organs of the lamprey. 23 



culum obviously correspoDds with those which form the sclerotomes 

 in the pronephric region (see Fig. 39 scl) in a somewhat younger 

 stage. The lateral mesoderm in Fig. 37 is clearly separable into 

 somatic and splanchnic layers [som and spT). The former layer has 

 a small, pointed diverticulum in the region serially homologous with 

 the pronephric diverticulum of Fig. 38 {pron). In some embryos, I 

 have seen at this point a diverticulum even more closely resembling 

 an incipient tubule than that represented in Fig. 37. This figure was 

 drawn to show the unusually clear development of the sclerotome from 

 a diverticulum of the myotome. 



d) The further Development of the Pronephros. 



The last stage of the pronephros considered was represented in 

 sagittal section in Fig. 41. Fig. 40 represents a similar section from 

 a somewhat older embryo, in which the collecting duct {cd) is not 

 distended with fluid as in Fig. 41. The five tubules, too, are longer 

 and more slender and the coelom (pericardial cavity) has increased 

 in size. The section necessarily passes to one side of the median 

 plane, so that the tubular heart, lying in the pericardial cavity, is 

 not cut. The liver diverticulum {int)^ however, is seen just under the 

 most posterior tubule of the pronephros. Between the pronephric 

 duct and the chorda the sclerotome cells are increasing rapidly by 

 division , while the peritoneum continuous with the orifices of the 

 nephrostomes is spreading. Between the funnels, too, are seen a few 

 trabecular cells with long protoplasmic processes passing from tubule 

 to tubule thus dividing the intertubular spaces, which do not com- 

 municate with the body cavity, into still smaller spaces. 



Figs. 28 and 29 are from cross- cections through the pronephros 

 of two embryos in stage 4. Fig. 28 has already been described in 

 connection with the temporary distention of the collecting duct. On 

 the right side of Fig. 29, a pronephric tubule has been cut through 

 its entire length. The section shows in the development of the 

 sclerotome several interesting features not seen in the younger embryo 

 represented in Fig. 28. The sclerotome cells have migrated up in 

 single file on either side between the chorda and neural tube on the 

 one hand and the myotome on the other. The intestine with its 

 envelope of splanchnic mesoderm has moved away from the chorda 

 leaving a space which will become the aorta. Along the roof of this 

 space runs the hypochorda ^). 



1) The hypochorda, a well-known structure in the embryos of 

 the Ichthyopsida, has recently been made the subject of special investi- 



