MUSEUM OF COMPAEATIVE ZOOLOGY. 209 



Stage IL 



Plate I. Figs. 4, 5. Plate II. Figs. 13, 14. 



This stage includes embryos with a distinct medullary groove, the 

 edges of which, however, have not yet fused to form a complete neural 

 tube. Several protovertebrse can be distinguished. 



In ti'eating of the structure of the pronephros in this stage I shall first 

 consider two embi-yos, which, judging from external appearances, seem to 

 have reached the same stage of development. These embryos are about 

 as far advanced as the one figured by Hertwig ('83, Taf. V. Fig. 6). In 

 both the medullary groove is widely open. They are about 2.5 mm. 

 long, and have been sectioned, one transversely, the other frontally. 



Following the series of cross sections forward from the tail end, and 

 comparing them with those of the preceding stage, the changes which 

 have occurred will be apparent. In the posterior region, the mesoderm, 

 as it passes outward and downward from the chorda, tapers much more 

 rapidly than in the earlier stage. Even as far posteriorly as a few sec- 

 tions in front of the blastopore, this condition can be observed ; and, in a 

 section 0.72 mm. from the posterior end, the thick central mass of meso- 

 derm, the protovertebral region (Fig. 4, la. 2Jr'vr.), has a triangular out- 

 line in cross section, and is readily distinguishable from the lateral plate 

 (la. I.), with which it is continuous at its outer angle. The protover- 

 tebral plate consists of an outer epithelial layer and a central mass of 

 cells. It is the former which is prolonged into the lateral plates. Each 

 of these is here in general only one cell deep. Between somatopleure 

 and splanchnopleure a few scattered cells occur, which can be assigned 

 only with difficulty to either layer. 



At 0.96 mm. from the posterior end the hindermost protovertebra 

 visible in cross section can be distinguished. Between this point and 

 the ganglion nodosum four protovertebrse are to be observed. Passing 

 farther forward, it is difficult to assign boundaries to the protovertebrse. 

 There is certainly one which is partially broken up into mesenchymatic 

 tissue.^ Still farther forward the series of the protovertebrse is con- 

 tinued by mesenchyme of a yet looser structure. Inasmuch as I have 



^ I use this expression merely as descriptive of tissue of a certain histological 

 character, quite independently of its origin. Indeed I am convinced, from observa- 

 tions which appear in tlie sequel, that not merely the head mesenchyme, but also 

 much of that in the trunk, arises in relatively late stages from mesodermal tissue, 

 substantially in accordance with the account of Balfour (78, pp. 107 et seq.), which 

 has recently found champions in Ziegler ('88) and others. 



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