834 SUMMARY OP CUKRENT RESEARCHES RELATING TO 



primary length of • 04 mm. to that of half the adult necessitates a 

 remarkable provision for nutrition. When the newly fertilized ovum 

 enters the narrow portion of the uterus it becomes almost entirely shut 

 off from the rest of the cavity into a " brood-chamber " by the 

 thickening of the connective tissue of the wall above and below it ; 

 the brood-cavity widens by the thinning out of its epithelium at the 

 points of contact with the ovum, now segmented. The latter at this 

 stage is a hemispherical mass of cells, attached by its broad base ; in 

 it a small cavity appears and increases in size. The basal cells, 

 which have a long, narrow, dense nucleus, multiply, close up the 

 opening of the hemisphere formed by the embryo, and unite the latter 

 to the uterine epithelium ; the remaining cells have a large, roundish, 

 granular nucleus, and by their multiplication cause a thickening of the 

 free wall of the embryo. This now has the form of a compressed 

 sphere, the long diameter being • 09 mm., the short one • 07 mm. 

 The basal cells increase, some in breadth, some in numbers, and form 

 an embryonal placenta ; they also give rise to a very delicate mem- 

 brane, the amnion, which invests the embryo and is closely applied 

 to the epithelium of the uterus. 



Meanwhile the epithelium of the brood-cavity has developed a 

 number of dark pigment-granules in its flattened cells. The placenta 

 grows and forms a solid stalk for the embryo, which is now solid 

 owing to filling of the segmentation cavity by cells from the free 

 (ventral) side, at a point which represents the blastopore, and from 

 which material long continues to be proliferated off into the embryo 

 to form the inner germ-layers. The intestinal cavity originates 

 by splitting of the central mass of cells ; and the attached side of 

 the embryo is visibly differentiated into endo- and ectoderm, the free 

 side still appears undifferentiated. The epithelium of the brood- 

 cavity increases in thickness, the outlines of its cells disappear, the 

 neighbouring connective tissue exhibiting a lacuna which is perhaps a 

 haemal cavity. The next stage is termed the " fungiform," the 

 embryo growing laterally outwards from the summit of its pedicle ; 

 it exhibits decided bilateral symmetry ; as seen from above it is oval, 

 rather broader at one (the head) end than at the other, and near the 

 narrower end exhibits a shallow depression, bounded towards the broad 

 end by a low wall, and representing on the ventral side the point of 

 proliferation inwards of the mesoderm and endoderm. The former is 

 detached forwards from the ectoderm as a compound layer, and lies 

 between the two other layers ; passing off posteriorly into the undif- 

 ferentiated cell-mass, it receives constant additions from the ectoderm. 

 The uterine epithelium continues to thicken and forms a central ring 

 which divides the brood-cavity into two halves ; the amnion is well 

 developed and consists of numerous cells with large nuclei. After 

 further increase in length of the embryo the primary anus appears as 

 a slit in the median line in front of the point of intra-proliferation of 

 the ectoderm, the primary mouth much further forward, as an invagi- 

 nation of a few cells of the ectoderm, which penetrate obliquely forwards 

 into the intestine as a solid mass, becoming hollow later. Segmenta- 

 tion commouces by the formation in the oldest part of the mesoderm 



