414 A HUMAN EMBRYO OF THE PRESOMITE PERIOD. 



the amniotic vesicle. The primitive groove and allantois are present. In the wall 

 of the yolk-sac are cell groups that seem to be blood-islands, but no definite vessels 

 were found in the body-stalk or chorionic membrane. 



The two ova described respectively by Reichert (1873) and Rossi Doria (1905), 

 both of which are frequently referred to and which are about the same size as those 

 in this group, can not, however, be used for this comparison, as no description of the 

 embryo is given by either of the authors. 



A summary of the features occurring in all of the embryos of this group would 

 include the presence of a primitive groove and an allantois, with evidences of blood- 

 vessel formation in the wall of the yolk-sac and (in most of them) also in the body- 

 stalk and chorionic membrane. Furthermore, in all of them the yolk-sac is con- 

 siderably larger than the amniotic cavity. The chorion is covered with freely 

 branching villi and its ectoderm is clearly differentiated into two layers. The meso- 

 derm lining the chorion forms a well-defined supporting membrane, from which 

 processes extend as the cores of the villi. The internal diameter of the chorion of 

 such an ovum is from 4 to 6 mm. 



All of these characteristics are present in the Mateer specimen. In addition, 

 the latter, as we have seen, also shows evidences of blood-vessel formation in the 

 villi, which was not reported in the others. It may be that the Mateer ovum is 

 more advanced in development and is thus on the border-land between groups 

 2 and 3. On the other hand, it is apparently in a better state of preservation than 

 the others of this group, a factor of great importance in the recognition of the early 

 stages of vasculogenesis. In the other specimens the process may have been 

 obscured by the poor preservation of the tissues. 



GROUP 3. EMBRYOS HAVING MEDULLARY FOLDS AND A NEURENTERIC CANAL. 



The first three specimens to be mentioned in this group are those described 

 respectively by Grosser (1913), Strahl (1916), and Ingalls (1918). These resemble 

 each other very closely, both in form and in size, and all are in a state of good preser- 

 vation. In each the greatest external diameter of the chorion is about 10 mm., the 

 greatest internal diameter about 8 mm., and the yolk-sac nearly 2 mm. in its 

 greatest diameter. The only marked difference in size is found in the embryonic 

 shield, which in the Grosser and Strahl specimens is slightly less than 1 mm. long, 

 while in the Ingalls specimen it is 2 mm. long. In all of these embryos there is a 

 distinct head-process with its contained canal, 1 together with a completion-plate. 

 One can also now speak of medullary folds, and at the caudal end of the primitive 

 groove the ectoderm and endoderm unite in the formation of a well-defined cloacal 

 membrane. Thus, both in the differentiation of these particular features and in 



1 A complete description of this canal is given by Ingalls, who designates it as the archentfric canal, holding, con- 

 trary to Hubrecht and Keibel, that in the formation of the head-process all the essentials of gastrulation are repre- 

 sented, and that its lumen is in reality an archenteric canal. Furthermore, according to him, the frequently used 

 term chordal, or notochordal canal is inadequate, since only a part of the wall enters into the formation of the chorda. 

 Similarly, an objection might be raised to the use of the term neurcnleric canal as a designation for the canal in its 

 earlier form, as in the strict sense this can refer only to the caudal portion of the canal of the head-process the short, 

 sharply defined canal connecting the caudal end of the floor of the medullary groove with the gut, as is best seen in 

 the older specimens of this group. 



