MAMMALIA. 181 



the middle layer become columnar, and form the columnar epiblast present 

 in the embyonic area at the end of the sixth day. The mesoblast first takes 

 its origin in the region and on the formation of the primitive streak. 



The investigations of Heape and myself do not extend to the first for- 

 mation of the intermediate layer found on the fifth day. We find on the 

 sixth day in germinal vesicles of about 2 - 2 '3 '5 millimetres in diameter 

 with embryonic areas of about - 8 nun. that the embryonic area (fig. 136) 

 is throughout composed of 



(1) A layer of flattened hypoblast cells ; 



(2) A somewhat irregular layer of more columnar elements, in some 

 places only a single row deep and in other places two or more rows deep. 



(3) Flat elements on the surface, which do not, however, form a con- 

 tinuous layer, and are intimately attached to the columnar cells below. 



Our results as to the structure of the blastoderm at this stage closely 

 correspond therefore with those of Kolliker, but on one important point we 

 have arrived at a different conclusion. Kolliker states that he has never 

 found the flattened elements in the act of becoming columnar. We believe 

 that we have in many instances been able to trace them in the act of 

 undergoing this change, and have attempted to shew this in our figure. 



Our next oldest embryonic areas were somewhat pyriform measuring 

 about 1-19 mm. in length and '85 in breadth. Of these we have several, 

 some from a rabbit in which we also met with younger still nearly circular 

 areas. All of them had a distinctly marked posterior opacity forming a com- 

 mencing primitive streak, though decidedly less advanced than in the blasto- 

 derm represented in fig. 140. In the younger specimens the epiblast in front 

 of the primitive streak was formed of a single row of columnar cells (fig. 

 138 A), no mesoblast was present and the hypoblast formed a layer of 

 flattened cells. In the region immediately in front of the primitive streak, 

 an irregular layer of mesoblast cells was interposed between the epiblast 

 and hypoblast. In the anterior part of the primitive streak itself (tig. 138 B) 

 there was a layer of mesoblast with a considerable lateral extension, while 

 in the median line there was a distinct mesoblastic proliferation of epiblast 

 cells. In the posterior sections the lateral extension of the mesoblast was 

 less, but the mesoblast cells formed a thicker cord in the axial line. 



Owing to the unsatisfactory character of our data the following- 

 attempt to fill in the history of the fifth and sixth days must be 

 regarded as tentative 1 . At the commencement of the fifth day the 

 central thickening, of what has been called above the primitive 

 hypoblast, becomes divided into two layers : the lower of these is 

 continuous with the peripheral hypoblast and is formed of flattened 

 cells, while the upper one is formed of small rounded elements. The 

 superficial epiblast again is formed of flattened cells. 



During the fifth day remarkable changes take place in the epi- 

 blast of the embryonic area. It is probable that its constituent cells 



1 The attempt made below to frame a consecutive history out of the contradictory 

 data at my disposal is not entirely satisfactory. Should Kolliker' s view turn out to be 

 quite correct, the origin of the middle layer of the fifth day, which Kolliker believes 

 to become the permanent epiblast, will have to be worked out again, in order to 

 determine whether it really comes, as it is stated by Van Beneden to do. from the 

 primitive hypoblast. 



