George L. Streeter , 87 



rapid; for example, a 14.0 mm. human embryo shows a stage found in 

 the 18.0 to 20.0 mm. pig embryo. The preparation of these dissections 

 was facilitated by the use of the following method: The entire embryo 

 is mordanted for several hours in a solution of potassium bichromate 

 2.5 per cent, and glacial acetic acid 10 per cent, after which it is rinsed 

 in water and brought into 80 per cent alcohol. The embryo, after a few 

 minutes dehydration in absolute alcohol, is then attached with a drop of 

 thick celloidin to an isinglass strip, previously coated with thin celloidin. 

 Mica or isinglass is used because it can be easily cut to any desired size. 

 By smoking it, before the celloidin coat, it is possible to write any desired 

 label on it, and the black serves as a good background for the embryo. 

 The whole is then hardened for a short time 80 per cent alcohol, and it is 

 then ready for dissection. In order to hold the embryo during dissection 

 the isinglass strip on which the embryo is mounted is clamped to a stage 

 which is made by fastening a glass slide with balsam to one of the facets 

 of a cut-glass polyhedral paper-weight. Such a stage is steady and can 

 be placed in any desired plane. With the embryo firmly mounted in this 

 way the dissection is made under alcohol with a binocular microscope. 



Description of Embryos at Different Stages. 

 Emhryos of About Three Wcel:s. 



Hertwig Collection, jSTo. 134 4.0 mm. 



Mall Collection, N"o. 148 4.3 mm. 



(See Fig:s. 1, 2, 3, and Plate I.) 



By the twentieth day the structures have become definite enough in 

 outline to permit of reconstruction. At this stage the ganglion crest of 

 the after-brain and spinal cord has divided longitudinally into right and 

 left halves, each of which has migrated ventro-laterally along the neural 

 tube and forms a flattened cellular band extending caudalwards from the 

 auditory vesicle along the lateral border of the tnbe to its extreme tip. 



That part of the crest which corresponds to the spinal cord consists 

 of compactly grouped cells which are so arranged as to present a flattened 

 continuons portion or dorsal bridge, the dorsal border of which is rather 

 smooth and sharply outlined, and shows no connection with the neural 

 tube. There are as yet no dorsal nerve roots. Projecting ventralward 

 from this bridge is a series of rounded segmental clumps of cells which 

 form the primitive spinal ganglia. These end diffusely among the devel- 

 oping fibres of the ventral roots. The ventral border of the ganglionic 

 crest is ill-defined in contrast to tbe dorsal edge of the bridfje of the crest. 

 7 



