4.54 Lei and Griggs. 



plate, a condition very different from that shown in fig. 3, C. 

 These differences, although not very great, go to support the 

 position maintained in this paper that four different grooves 

 appear in the median dorsal line of the embryo of Amblystoma. 



Some account should also be given here of the histology of the 

 grooves. It has been assumed by many writers that there is 

 one primitive groove running over the dorsal surface of the 

 embryo showing throughout its length a fusion of the germ layers. 

 Miss Johnson ('84), however, has found that in the newt there is 

 a fusion of germ layers only in the vicinity of the closed blasto- 

 pore and that there is an ''apparent fusion" in the ''anterior 

 pit" as sne calls the deeper portion of the anterior germinal 

 depression. In Amblystoma the ectoderm and endoderm are 

 clearly separated in the region of the neural groove and of the 

 posterior depression out the condition in the anterior depression 

 and in the b astogroove is different. At the deepest part of the 

 anterior germinal depression the ectoderm is pressed down 

 against the endoderm (fig. 3, E). The fusion, however, is not 

 real but merely "apparent" or mechanical for there is a readily 

 recognizable difference in shape and contents between the two 

 types of cells, and there are no cells between the two layers 

 intermediate in character. In the case of the blastogroove 

 there is a real fusion of layers (fig. 3, F). At the posterior end of 

 the groove the cells can be followed through from the ectoderm to 

 the endoderm. There is no space between the two layers, and the 

 cells lying midway between the two are intermediate in character. 

 Toward the anterior end of the blastogroove, however, the 

 anlage of the notocord has differentiated and the two germ layers 

 have separated (fig. 3, G). A few small undifferentiated cells 

 may still be seen in the bottom of the anterior end of the blasto- 

 groove resembling those at the posterior end. 



The peripheral groove and neural crest in this stage are extend- 

 ing backward and meet around the posterio end of the neural 

 plate (fig. 2, A, B) . This process clearly defines the posterior 

 limit of the neural plate and it is seen that the anus lies outside 

 the neural plate in the region of the neural crest (a, fig. 2, B). 



