266 STUDY OF PIG EMBRYOS. 



in the embryo are all small branches of the veins when they first appear. Their 

 great enlargement does not occur until comparatively advanced stages. Finally! 

 attention should be paid to the following important modifications in the mesenchyma. 

 Already there has been a rich development of a plexus of fine blood-vessels over 

 the surface of both the fore- and hind-brain which has been accompanied by a 

 slight condensation of the mesenchyma between the blood-vessels, thus markirfg a 

 distinct membrane, in which we can easily recognize the pia mater, Pia. Outside 

 of the pia mater comes a relatively broad zone, Arach, in which the cells are 

 widely separated from one another and are connected by slender and long processes, 

 so that the intercellular spaces are very extensive. This broad zone is the' anlage of 

 the arachnoid membrane. It is much more differentiated around the ventral portion 

 of the brain than around the dorsal side. Between the arachnoid zone and the 

 external epidermis the mesenchyma is somewhat more condensed and the cells are 

 elongated in form, in part almost spindle-shaped, forming a layer, Cut, which we 

 may consider the anlage of the cutis, and perhaps, also, of the subcutaneous tissue, 

 but this is doubtful. Between the arachnoid zone and the cutis zone, so placed 

 that they cannot be quite said to belong to either one or the other, appear numer- 

 ous blood-vessels. These form a more or less distinct vascular layer, which ap- 

 pears with remarkable constancy in all classes of vertebrates, and over a large 

 part of the body. It may, therefore, be called the panchoroid. It is unquestion- 

 ably of very great morphological importance, but its history is imperfectly known. 



As regards the histological condition of the tissues, the student should make 

 careful observations. Attention may be directed especially to the following points : 

 The epidermis at the sides of the section is two-layered and consists of an inner 

 layer of cuboidal cells, the anlage of the Malpighian layer of the adult, and of 

 an outer layer of very thin cells, the epitrichium, the nuclei of which are flattened 

 and appear darkly stained. Toward th^ medLli line, above the hind-brain and 

 T5eTow T^c * Tore-brain, the epidermis becomes gradually one-layered and much thin- 

 ner. The mesenchyma exhibits three principal forms of cells: First, those which are 

 equally branched in all directions, and represent a primitive form of the tissue. 

 Such may be found in the neighborhood of the basilar artery. Second, the elon- 

 gated cells of the cutis zone; and, third, the cells of the arachnoid zone above 

 described. The blood-vessels have very distinct endothelial walls which are very 

 thin, being thickened only to furnish space lor the nuclei, which, unlike those of 

 the adult, project not only into the lumen of the vessel, but also against the 

 surrounding mesenchyma. The red blood-corpuscles are rounded cells, some- 

 's oval, not infrequently somewhat distorted. Their nuclei are nearly spherical 

 a number of fine granules. Mitotic figures are quite frequent. A few 

 i are beginning to char -.- ' v becoming smaller and taking the stain 

 -onipare page 94). rvous system the differentiation of the 



Vain is mo 1 - n in the fore-braip but even in the 



> Tve-cells and the young neuroglia 



