358 



FREDA M. BACHMANN. 



are easily recognized in any stage by their shape, size, and 

 staining qualities. 



EMBRYOS 3.2-3.7 MM. LONG. 



In embryos of this length the blastopore is closed and the tail 

 extends about I mm. beyond the yolk. The pronephric ducts 

 have been formed. There is no trace of a split in the mesoderm 

 which is to give rise to the coelomic cavity. The axial mesoderm 

 is a compact mass of cells extending laterally and ventrally and 

 composed of cells which are still quite regular in outline although 

 the nuclei are more or less oval. There is scarcely a trace of 

 yolk in these mesodermal cells. The entoderm cells frequently 

 contain large yolk masses or have yolk material diffused through- 

 out the cytoplasm. 



The germ cells are found in the lateral plate of mesoderm in the 

 region just anterior to where the tail leaves the yolk. The germ 

 cells may be anywhere in the lateral plate but the majority of 

 them were found some distance away from the mid-line (Fig. i). 

 Not infrequently they are far out in the lateral plate as shown in 

 Fig. 2. A few germ cells were found between the yolk and the 

 mesoderm. It may be possible that they came from the yolk 

 but at least none were seen in the yolk nor coming out of it. 

 The germ cells are so very different in appearance from the other 

 somatic cells that there is no danger in overlooking them in well- 

 stained sections. The length of the region in which they are 

 found varies from 0.025 mm. to 0.054 mm - The embryos are 

 not symmetrical with regard to the number of germ cells found 

 on each side. The total number of an embryo is from 12 to 28. 

 The following table shows the number found on each side, the 

 total number, and the distance throughout which the germ cells 

 were distributed in several of the embryos sectioned. 



