900 GENERATION. 



its surface, there appears a rounded elevation or heap of smaller cells, forming a distinct 

 spot, called the embryonic spot. As development advances, this spot becomes elongated 

 and oval. It is then surrounded by a clear, oval area, called the area pellucida, and the 

 area pellucida is itself surrounded by a zone of cells, more granular and darker than the 

 rest of the blastoderm. The elongated line thus formed and surrounded by the area 

 pellucida is called the primitive trace. It has been shown, however, that this primitive 

 trace, or primitive groove, is a temporary structure and has nothing to do with the 

 development of the neural canal. After the groove is formed, there appears in front of, 

 but not continuous with it, a new fold and a groove leading from it. This is the " head- 

 fold," and the groove is the true medullary groove, which is subsequently developed into 

 the neural canal. If we adopt this view and the difference is not very important we 

 simply substitute the new trace, which is the seat of the development of the neural canal, 

 for the original primitive trace, which is temporary. It is probable that embryologists 

 have heretofore noted the so-called primitive trace and studied subsequently the develop- 

 ment of the true medullary groove, supposing that they were identical structures in dif- 

 ferent stages of formation, and not observing that the first trace disappears. 



Blastodermic Layers. Shortly after the appearance of the primitive trace, the blasto- 

 dermic cells, which are at first arranged so as to form a single membrane, separate into 

 layers. These layers have been differently described by various observers, and there is 

 some uncertainty with regard to the application of direct researches made upon the chick, 

 in which most of these early processes of development have been studied, to the mam- 

 malia and the human subject. We shall endeavor to describe the different layers in as 

 simple a manner as is consistent with our positive knowledge, omitting all points that are 

 unsettled or which seem to be of minor importance. 



The blastodermic cells, resulting originally from the segmentation of the vitellus, are 

 first apparently split into two layers, which may be termed the external and the internal 

 blastodermic membranes. According to the most recent observations, the main portion 

 of the external layer, sometimes called the serous layer, simply forms a temporary invest- 

 ment for the rest of the vitellus and is not developed into any part of the embryon. The 

 internal layer, called the mucous layer, is developed into nothing but the epithelial lining 

 of the alimentary canal. There is a thickening of both of these layers at the line of devel- 

 opment of the cerebro-spinal system, with a furrow, which is finally enclosed by an ele- 

 vation of the ridges and their union posteriorly, forming the canal for the spinal cord. 



As the spinal canal is thus developed, a new layer is formed, by a genesis of cells from 

 the internal surface of the original external layer and the opposite surface of the internal, 

 or mucous layer. This layer of new cells may be termed the intermediate layer ; and it 

 is from this that nearly all the parts of the embryon are developed. 



To summarize the development of the layers just mentioned, we may state that the 

 external layer is a temporary structure ; the internal layer is very thin and is for the 

 development of the epithelial lining of the alimentary canal ; the most important structure 

 is a thick layer of cells, developed from the opposite surfaces of the external and the 

 internal layer and situated between them, called the intermediate layer ; and it is from 

 these cells that the greatest part of the embryon is formed. 



Formation of the Membranes. 



The brief description we have just given of the formation of the blastodermic layers 

 seemed necessary as an introduction to the study of the membranes ; and we shall defer, 

 for the present, the description of their development into the different parts of the 

 embryon. 



In the mammalia, a portion of the blastoderm is developed into membranes, by which 

 a communication and union are established between the ovum and the mucous membrane 

 of the uterus. From the ovum, two membranes are developed; one non-vascular, the 



