EMBRYO OE 12 MM. STUDIED IN SECTIONS. 173 



divisions between the mid-brain and hind-brain are marked by concavities in the 

 outline of the head. The eye is both absolutely and relatively larger. Above it 

 can be distinguished readily the anlages of the great bristles which develop 

 over the eye, corresponding to the human eyebrow. These anlages appear 

 as whitish spots, for they are thickenings of the ectoderm. The snout has 

 increased in length ; the external ear has grown longer and has begun to as- 

 sume its permanent pointed form. The limbs have increased considerably in 

 length, but not yet enough to pass beyond the line of the abdomen. In both 

 feet the differentiation of five toes is clearly indicated. The milk-line, as a line, 

 has almost completely vanished, but the row of toes, and the anlages of the 

 mammary glands, which develop along the milk-line, persist and will undergo 

 further development in later stages. The number in the specimen figured is 

 five. A row of these anlages marked the position of the milk-line, which 

 demonstrates that there has been a more marked growth of the dorsal region of 

 the body, and comparison of embryos of 15 and 20 mm., during the period com- 

 prised between those two stages, indicates that the growth of the dorsal part 

 of the embryo is far greater than of the ventral part. Comparison of figures 

 1 10 and 1 1 1 shows at once that the area occupied in both figures by the region 

 on the ventral side of the milk-line is about the same. In the pig of 20 mm. there 

 is no indication of the segmental structures recognizable in the surface modeling. 



Pig Embryo of 12 mm. Studied in Sections. 



A pig embryo of 1 2 mm. has been selected as the center of study in this book 

 because its anatomical relations are such that they may be readily grasped by 

 the student who has already studied the anatomy of an adult mammal, human 

 or other. At the same time the development of the organs is so advanced that 

 their fundamental relations may be observed. From an embryo of this size the 

 transition to the study of younger embryos is, even for the beginner, compara- 

 tively easy. It is not necessary that the embryo should be of this exact size; 

 indeed, it may be somewhat advantageous for the student to have an embryo 

 a millimeter larger, or one, two, or even three millimeters smaller, since the fig- 

 ures and explanations referring to the 12 mm. stage will enable him to identify 

 all the structures to be found and yet call upon him for the exercise of care and 

 judgment in identifying, from the data given in the following pages, the various 

 parts in the somewhat different stage he may be studying. The 1 2 mm. size was 

 chosen partly because the author had at his disposal three good series belonging 

 to the Harvard Embryological Collection. 



The transverse series is the most important, and should form the basis of 

 the study, and accordingly most of the sections figured are from such a series. 



