THE EMBRYOLOGY OF THE OPOSSUM 53 



edge of the relatively opaque disc (stage 15). This light spot, 

 which was first described by Hartman, is the area in which 

 the first mesodermal cells appear. 



Stage 16 



The primitive streak. When the vesicle is about 1.8 mm. 

 in diameter Hartman discovered that certain cells in the 

 peripheral half of a radial line through the center of the light 

 spot mentioned above, drop down beneath the medullary 

 plate and lie between the latter and the endoderm. These are 

 the first mesodermal cells. When enough of them have col- 

 lected in this region to form a slight local opacity they show 

 up in transmitted light as a dark streak in the light spot at 

 the point where the latter is nearest the edge of the medul- 

 lary plate (stage 16). This is the primitive streak. It is the 

 only part of the vesicle which is 3 cell layers thick. 



It may seem strange on first thought to find the primitive 

 streak arising within the medullary plate. The process of 

 'gastrulation,' or its equivalent, began back in the fourth day 

 with the origin of the endoderm. The primitive streak stage, 

 as is always the case, represents a late stage of gastrulation, 

 and it is not surprising at a late stage to find the equivalent 

 of the blastopore within the medullary plate. Such a situa- 

 tion is universal, and is, of course, the explanation of the 

 neurenteric canal in those forms in which one occurs. 



From this time (stage 16) right on through the end of 

 the tenth day (stage 29) the primitive streak will be perma- 

 nently recognizable in the posterior growing tip of the medul- 

 lary plate the sinus rhomboidalis. During stage 30, the 

 first half of the eleventh day, the posterior neuropore will 

 finally close over, and the primitive streak will be lost in the 

 floor of the spinal cord of the tail. 



The origin of the first mesodermal cells is the point at which 

 Hartman 's studies of the early embryology of the opossum 

 came to an end. An exhaustive account of the origin of these 

 cells, the formation of Hensen's node, and the spreading of 

 the mesodennal crescent, is in preparation by Dr. C. II. 



