228 



STUDY OF PIG EMBRYOS. 



becomes open, differing in this respect from all the others. Why it has this pecu- 

 liarity we do not know. The opening does not persist, but the exact history of 

 its closure is at present unknown. The process, Hy, described as shutting in the 

 external portion of the second gill cleft has sometimes been termed the opercu- 

 lum, because it covers a gill cleft opening, as does the operculum of a bony fish. 



Ec. mes. 



Fig 133. — Pig, 9.0 mm. Frontal Series 54, Section 459. 

 A.bas, Basilar artery. Ao.d, Descending aorta of the left side. Ao.j, Third aortic arch. A0.4, Fourth 

 aortic arch arising from the median ventral aorta. cl.II.ex, External portion of the second gill cleft. 

 cl.III, Third gill cleft. D.Z, Dorsal zone of the medulla oblongata. Ec, Ectoderm. Epen, Ependymal 

 roof of the hind-brain. Hy, Hyoid arch. Jug, Jugular vein, mes, Mesenchyma. Ot, Posterior wall 

 of the otocyst. P.Ao, Pulmonary aorta. Ph, Pharnyx. Sovi, Somatopleure. V.Z, Ventral zone. 

 X 22 diams. 



In the lower part of our figure a portion of the somatopleure, Som, is shown where 

 it extends ventralwards to form the wall of the pericardial cavity. There is also 

 included in the drawing a part of the pulmonary aorta, P. Ao. 



Pig Embryo of 6 mm. 



Of this stage two transverse sections are figured in order to give more exact 

 notions as to the structure of neuromeres and the secondary segments. The first 

 section is taken through the level of the head, and may be directly compared with 

 figure 113. The relations are so closely similar that it is unnecessary to describe 

 the present section (Fig. 134) in detail. The explanation of the figure is suffi- 



