TRANSVERSE SECTIONS OF EMBRYO OF 12 MM. 181 



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 

 appears 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 unques- 

 tionably of very great morphological importance, but its history is almost un- 

 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 the median line, above the hind- 

 brain and below the fore-brain, the epidermis becomes gradually one-layered 

 and much thinner. 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 elongated cells of the cutis zone; and, third, the cells of the arach- 

 noid zone above described. The blood-vessels have very distinct endothelial 

 walls which are very thin, being thickened only to furnish space for the nuclei, 

 which, unlike those of the adult, project not only into the lumen of the vessel, 

 but also against the surrounding mesenchyma. The blood-corpuscles are 

 rounded cells, sometimes oval, not infrequently somewhat distorted. Their 

 nuclei are' nearly spherical and contain a number of tine granules. Mitotic 

 figures are quite frequent. A few of the nuclei are beginning to change by be- 

 coming smaller and taking the stain more deeply (compare page 94). In the 

 nervous system the differentiation of the cells in the hind-brain is more advanced 

 than in the fore-brain, but even in the hind-brain the distinction between the 

 young nerve-cells and the young neuroglia cells (neuroblasts and spongioblasts) 

 is not very clear. The nuclei are only just beginning to acquire distinct nucleoli, 

 such as would be characteristic of later stages. The nuclei of the tissues differ 

 markedly from those of the earliest embryonic stages, but can scarcely be said to 

 have assumed in any of the tissues adult characteristics. 



Section through the Lower Part of the Otocyst. Figure 1 1 5 is from section 198, 

 and, therefore, ten sections below figure 113. It is inserted chiefly to bring out 

 three points not shown in the preceding illustration : First, the root of the spinal 

 accessory nerve, N. //, which arises from the cervical (in the figure upper) end of 

 the hind-brain and runs forward to join the vagus ganglion, N. 10 jug, the jugu- 

 lar ganglion cf the adult. Second, the characteristic relations of the jugular vein 



